I read this question on another board but it hasn't been answered and it interests me too: can you edit FR so much that any saves you have won't work with it anymore?
Yes. The simplest thing I can think of would be making you get stuck when you open the save file by remapping the area you're saved in.
But since you probably wanted the answer for a situation if the save file can get corrupted when you try to continue playing it within the game, the answer to this is also yes. It's practically possible to do anything with the game by not only changing some data used by the engine (maps, scripts, all kinds of table entries, and such), but also how the engine itself works. If you try creating an IPS patch that uses pokemon firered as the unmodified rom file and new modified rom file as some Mario game for GBA, you should be able to create a patch file between them. What this means is that the patch file then contains information behind, how to change pokemon firered into a GBA Mario game.
And save file for FireRed "shouldn't" (= will not) work with that game.
Generally, it's possible to do
anything, but a good question that often comes into mind during the hacking process is "How?".
Cool! I have HxD, so how would I go about doing that?
If that one happens to have this functionality, you can "search and replace" all byte entries matching with text string "Rocket", or "ROCKET" (whatever it is mentioned as in the game) with another 6-mark text string you like. If you want to have a shorter, or longer name for your evil team, I can't think of a way to do it directly like this.
To search for those names ("Rocket", or "ROCKET"), you'll have to type those words in byte form as the
text table shows you:
https://datacrystal.romhacking.net/wiki/Pokémon_FireRed_-_TBL
Doing this kind of stuff
might lead into bugs though but it's unlikely that would happen. The reason for this is that if there is some data in the game that is not used as text and it's "text representation" would still be a byte sequence that would match with "Rocket", or "ROCKET", you would with this "search and replace" functionality also overwrite that with your new name. Since here the new name wouldn't be used as text, doing this would actually broke something. But this is very unlikely.