[RPG] Is a +1 chain shirt strictly inferior to a nonmagical breastplate

RAW there really is only one option to quickly take off armor: have someone help you.

Getting Into and Out of Armor

The time it takes to don or doff armor depends on the armor’s category.

Don. This is the time it takes to put on armor. You benefit from the armor's AC only if you take the full time to don the suit o f armor.

Doff. This is the time it takes to take off armor. If you have help, reduce this time by half.

The fastest a PC could shed heavy armor with help would be in 2 and a half minutes or 25 rounds of combat.

RAW options to avoid the damage

1. Run away. As I interpret the spell you would still need to be in its range to use the bonus action attack. Running out of range would be difficult to achieve since the spell attack is a bonus action allowing the caster to use both their actions to follow you. However if allies impede the movement of the caster or your move speed is greater it could be accomplished in one round.
2. Deal Damage. Every time the magic user takes damage while concentrating on a spell they are forced to make a CON save to keep that spell up. If everyone hits the wizard odds are pretty high that he will fail one of those CON saves.

Houserule option

Let the PC take it off with a standard action, but doing so damages the armor, preventing them from wearing it again until they have it repaired.

Musings on Heat metal

There is no requirement for a PC or NPC whose gear is affected by heat metal to react taking off their armor. The stipulation of the spell is to throw it if they can, donning and doffing seems to be a bit more than all that and as such I would interpret it to mean weapons should be thrown, but armor can be kept on. I know 2d8 bonus action damage sounds like a lot, but dice only damage has a way of only coming out to be around the average most of the time, while 9 damage a round without an attack roll is pretty nice, it is probably a better idea for the party to focus-fire on the wizard to end it rather than someone in heavy armor taking their armor off and seriously lowering their AC for the rest of the fight.

Long story short, you cannot. All ways to "modify" it will also involve permanently removing all magic from the item.

Most magic dispelling effects are temporary. A Disjunction spell will destroy the item completely. And breaking it until all hit points are lost will remove all magic, but the item can be fixed with spells and remain a simple masterwork item, meaning that it's much easier to simply buy another masterwork item.

As for mundane items, you will have to ask your GM, as there are no rules for this on the Craft skill. And most modifications will be a case by case analysis.

Like, attaching a chain to a kama will make a kasurigama, attaching another axe head to a two-handed axe will make a double-axe. If you make a pointy tip with metal, bone or otherwise and tie it on top of a quarterstaff, you could call it a spear. And so on.

But a lot of items will simply not work that way, and each GM/table will have different opinions on each item individually. Like, what is the difference between a composite bow +1 and a composite bow +4? What makes one stronger than the other?

I would say "everything", the craftmanship is completely different, you need a different wood, a different technique, etc. But some other guy might understand more about crafting of bows and say "no, all you have to do is work on it for a little longer and it will be studier". So this will suffer from table variation.

This kind of modification could easily fall under the Repair Item rules, as the base item isn't changed and is simply being modified, and as such:

• Pay 1/5 of the cost of that new item.
• Make a craft check against the same DC to create the new item.
• If you fail by 5 or more, the cost paid in materials is wasted and you must try again.

You might consider the difference between the two items, if they are too different (new item is more expensive than the base item), you might consider that you must make a new item altogether, and use the base item cost as material cost instead.