Minecraft – Will New Features Appear in Existing Worlds?

"How abrupt" depends entirely on the particular chunks in question, and is impossible to predict to boot. There's really no way to know until you try for yourself.

If you're unsure you want the potentially very obviously misaligned chunks, you can always backup your world before deleting it, reverting if the border does not line up particularly smoothly.

Unfortunately since the level generation algorithm has changed, using the same seed as before will generate a completely different map in its place. There are new biomes, their sizes are different, and the land features are different too. Since my friend and I have structures we would like to save, we're faced with the same problem you are.

It is nearly impossible to get a world that will match your existing one, and if your structures are like ours, it depends heavily on the land it was built on. Maybe you can paste in a whole island at once or something, but even then the biomes won't match (if you care about appropriate weather in the area you built it).

This is admittedly an opinion here, but my advice would be to start fresh; turn a new page. The Adventure Update is for exploring! Go have fun with the new features, and save your maps elsewhere for nostalgia. The update supports old maps, so you can always go back to see them and build upon them.