Easiest way to represent a NPC getting an upgrade?

Suppose-- no, specific example. I'd like to have an enemy type that, midway through the fight, gets an upgrade that gives it new attacks and increases its AC. I'm trying to think how best to do that - obviously having API access would be golden but I don't think just that alone is worth getting a plus for, and opening each token and changing it to the other type in the 'represents'.... well it's a bit slow since I'd have to do that for every enemy. I'm curious if there's some trick I'm not thinking of.

Your easiest solution would be to create your NPC then copy that sheet a number of times to equal the upgrades... when you need to make a change you can just change the token to represent the v2, v3 etc sheet instead and it will benefit from the changes to attributes or journal abilities. To preserve HP you'll want to leave bars unlinked but that is a minor issue.

It's not optimal (I'd have to open up the token and track down the upgrade each time) but it's not a terrible solution, I just wanted to see if there are other ways. I know API would let me make macros that just... change it. And I leave hp bars unlinked anyway for npc tokens

With the API you could change what the token represents with a macro but to be honest that is massive overkill compared to double clicking a token, clicking 'represents' and having your upgrade sheets named so that they are easy to find. If this is something you are likely to do often then the API is a great tool for all of these kind of things. For a one off, I usually try to think of how I'd handle it at a tabletop, in this case I'd have a stack of sheets and flip through them as needed per upgrade... basically the same as relinking a token to another sheet :) Remember, when setting a token's represented sheet you can open the drop down and start typing the name of the sheet you want and it'll get you in the general area... I'd suggest a 3-5 second change per upgrade once you know what/how and where  ¯\_(ツ)_/¯

You could have a copy of all the tokens linked to the other character sheet on the GM layer, then when you want to bring them in, click the tokens on the map layer and move them to the GM Layer and move the ones on the GM layer to the map layer...

also not a bad option but I think changing what they're tied to is a quicker method. It overrites (haha wow way to typo 'overrides' and 'overwrites') their names but those are hidden from the players anyway, and it's no scrolling if I name them [mobname] (roused) and [mobname] (vacuous). And a day I get to use 'vacuous' on is a good day.

I leave a border of Fog of War around most maps. Perfect for sticking things like extra monsters, images you want to be able to show to the players with a shift-Z command, etc. Just create a character token combo for each stage. When the buff happens, just switch tokens, pulling the old one off into the masked area, and the new one into the revealed area. Just two token moves, and no mucking about with settings. Keep It Simple.