I was thinking about using a unique (player only custom race) body replacer and I was wondering whether it's possible to make already existing armors to switch to a new mesh when worn by a player. Kinda like male<->female transition but within the same gender.
Would that require a script, something like:

check the condition of armor a1
remove armor a1
add and equip armor a11
set condition of armor a11

Is there a mod that already did something like this?

Yeah, I think it is possible. JoshNZ and DoctorL have been talking about a "new school" female body replacement project where NPCs have more varied bodies and corresponding outfits. I'm not sure how well it works as I have not tried the mods. As they contain nudity, I cannot link directly to them, but the mods can be found on nexus. Search for "Body Control" and "Female Enhancement Mod".