Unlike ck2 I haven't seen characters get rid of any health traits other than the sicknesses. I believe you would have to acquire something that conflicts with it, such as "eager reveler," but I haven't actually seen it happen with adults. Unfortunately, eager reveler does not get rid of the melancholy trait.
There are options to change character weight. Add 1/5/10 Character Weight. Remove 1/5/10 Character Weight. Weight is scaled from -100 to +100. On game start characters' weights are around 0. These options allows to make character thinner or larger. WEIGHT DOES NOT AFFECT MUSCLES GAINED FROM PROWESS. The same thing also proves true with the remove_trait, add_modifier and remove_modifier commands. Character: remove_trait Removes a trait from the character Character: add_modifier , Adds a modifier to a character. Input -1 for days to have the modifier become permanent. Character
7 items. Description. You can add/remove traits to/from your or other characters. Add/remove resources like piety/prestige/renown to/from your character or other characters. Enable/Disable Debug Mode without -debug_mode launch option. Change your or other character's culture. Change your or other character's religion. Spawn Characters.

There are two ways of losing incapable - the first being a demon child recovering from brain damage, which you likely don't apply for - the second is the "wake up as a lunatic" event. For the lunatic event, you need to be less than 65 years old, but the change of the event happening is extremely small unless you have the trait "syphilitic" or

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crusader kings 3 remove trait