@sft1965 @Nightmare2048 that’s interesting, they changed it in the latest update so that, even if a hero is locked into a costume (or non-costume) in Raid/War defense, you can change the costume elsewhere (e.g. Raid offense or class quest)
source: v28 release notes and question 9 of costume FAQ thread
My Joon is costumed in my raid defense, but I was able to change him out of the costume to use in this trial.