Minecraft – Update existing Minecraft java world to 1.13: What to expect

• Pretty much all commands break, since the entire command system was overhauled. With some automatic converters you can upgrade them or alternatively replace them manually with better solutions that are new in 1.13. Most changes can be found here (archive), but that post was made before the changes were done and there are some additional changes. But the new autocomplete is very good and mostly explains itself and the wiki was over time also updated, it should be accurate again now.
• The data pack structure has changed and of course resource packs also have to be updated to include the new blocks, entities, etc.
• Customised worlds don't work in 1.13. They can't be created, but they can be opened. When opened in 1.13, any new chunks generate like in a default world. The "buffet" world type replaces customised worlds, but it's neither completely done yet, nor does it have all features of customised worlds in 1.12. It's planned that they return later, but there's currenty no date or version known for that.
• There are a few other world generation changes, mostly to oceans. They might cause a few visible chunk borders, but nothing major. Existing chunks will not change, like in every update. The same seeds still work, even ocean temples should usually still generate in the same places.
• Mob systems that drown zombies to kill them don't work anymore, because they are converted into "Drowned" now.
  All undead mobs sink in water.
  Baby zombies burn in daylight.
• Water physics have changed a lot. In theory this shouldn't break existing builds, but there might still be some bugs with edge cases left.
  Items and XP orbs float up in water.
• You should wall yourself in for AFK farms now, Phantoms spawn when you haven't slept for at least 3 nights in a row. They fly, but unlike vexes and dragons, they don't fly through blocks. Having a solid block above you prevents them from spawning and cats scare them away. Alternatively you can use the command /gamerule doInsomnia false to turn off phantom spawning completely.
• Beds break if they have no supporting blocks under them. Since they were non-functional without support before anyway, this should only affect decorative beds.
• TNT that explodes when mined was removed and turned into regular TNT in 1.13.0, then readded in 1.13.1. If you skip 1.13.0, they should upgrade properly.
• And most importantly: Because there was a deadline for 1.13, it was developed pretty hastily and there were still many bugs, some quite game-breaking. There's also a lot of lag, both on the server and on the client.

In general I would not recommend updating to 1.13 at all. Instead, you should stay in 1.12.2 or, if you want the new features, directly update to the latest version. The later, the better.

The updates after 1.13.0 have continuously improved on the bad state that the game was left in, so that now, in 1.15.2, "only" four issues that I would consider critical are left:

• https://bugs.mojang.com/browse/MC-147818: Chunk loading is very messed up. Basically every bit of machinery can break randomly when you go away, come back or when you exit or join a world while being nearby. The best workaround is /forceload-ing the area and running the world on a permanently running server. Yes, wasteful, I know.
• https://bugs.mojang.com/browse/MC-108469: Entities can sometimes get deleted or duplicated, this has something to do with chunks loading and unloading and entities moving between chunks. There is no full workaround for this, but as long as an entity never crosses a chunk border, it should be completely safe from getting deleted.
• Customised worlds generate new chunks like a default world. The best workaround is to figure out how far chunks have already been fully generated, then set the world border to as much less than that as the render distance can at most be (in Singleplayer 32×16=512, on a server whatever you set in options.txt times 16), so that no new chunks are generated. When customised worlds finally get re-added, you will likely have to change level.dat back to a customised world, so create a backup of that file before opening the world in 1.13+.
• https://bugs.mojang.com/browse/MC-147729: Items can get deleted when you craft with the recipe book and pick up new items while doing that.

If you skip 1.13 and directly update to 1.15, there are of course additional changes to consider from 1.14 and 1.15, the main ones that impact an existing world are these:

• Some commands, datapack and resource pack related things have changed again in 1.14 and 1.15, but not as drastically as in 1.13. They still require updating. I recommend the technical changelogs by Slicedlime for details about these changes: 1.13 commands, 1.13 other technical changes [link pending], 1.14 and 1.15
• OpenGL 2.0 is now required, meaning that you might have to update your CPU drivers or, if you have a very old CPU, you might not be able to play Minecraft 1.15+. But this should not affect any reasonably modern computer, OpenGL 2.0 was released in 2004.
• Pretty much everything related to villagers has changed, that means that existing iron farms, trading halls, villager breeders and so on will very likely not continue working in 1.14+. There are lots of tutorials online on how to build new versions of these. In short: Beds are the new doors and work stations are important as well.
• The textures were updated to the new pack that was available for a while already. The old textures are available in the resource pack menu.
• 1.14 also had a few changes in world generation (mostly villages and jungles, which got a new variant "bamboo jungle"), so in some places you might see borders between old and new chunks. The same seeds still work, villages should even still usually generate in the same spots, but also generate additionally in biomes where they previously couldn't.
• If you had your world opened in the snapshot 19w36a at some point, then drowning, falling and fire will not damage you anymore. This was caused by the corresponding gamerules being set to the wrong value by default in only this snapshot. To fix this, run these three commands:
  • /gamerule drowningDamage true
  • /gamerule fallDamage true
  • /gamerule fireDamage true
• New mobs called "pillagers" spawn. If you kill a "pillager captain" (the one with a banner on its head), you get an effect for one hour that starts a "raid" when you enter a village. These raids are currently very broken, causing a pretty much unstoppable flood of monsters that attack the villagers and you. The best workaround for this is drinking milk to get rid of the effect.
• Ocelots no longer turn into cats when tamed. Instead, cats are their own mob.
• Vindicators can now break doors.
• Sticks are dropped by leaves sometimes, so if your tree farm assumes that you only get saplings and wood, you should consider that.
• Bees are now required for the advancement "Two by two" and honey bottles for the "A balanced diet" advancement, meaning you have to go out into newly generated chunks to get those, if you don't have them already.
• The burn time of scaffolding and boats as furnace fuel was increased.
• End stone bricks have a higher blast resistance.
• Combining tools, weapons and armour in the crafting table no longer works, this task was moved to the new "grindstone" block. Of course you can still use the anvil.
• Lapislazuli, cocoa beans, bone meal and ink saks now have to be crafted into dyes before they can be used as such.
• A few crafting recipes were changed.
• Nether portals can now link to portals on top of the bedrock ceiling of the Nether.
• Dispensers can now shear sheep with shears, get water into bottles and place armour stands. I don't really expect this to break anything, but it could in theory.
• A ton of bugs were fixed, some you might have relied on. The full lists are here: 1.14 (archive) and 1.15 (archive)

I do not recommend playing 1.14 anymore, it has horrible client performance and a lot of severe bugs. 1.15 improved on this a lot, but there are still the four big issues mentioned above left. I still recommend staying in 1.12.2 unless you either absolutely need the new features or you know all the details of these bugs and can work around them (which is very complicated and in some situations not possible).

I did not mention any new features that do not affect existing areas in existing worlds, but of course they are worth considering as well, for example you will encounter beehives, jungle forests and cats when you go exploring. But listing all of those features would turn this post into a complete changelog. For that, I recommend reading the changelogs on the Minecraft wiki:

• https://minecraft.gamepedia.com/Java_Edition_1.14 (archive)
• https://minecraft.gamepedia.com/Java_Edition_1.15 (archive)