Today I changed from Lv 42 Lancer to Lv 23 Carpenter without a problem. My Carpenter was missing bracelets because I have converted them yesterday and purchased upgrades but had not equipped them yet. Normally when something is missing, you equip the set and something is simply missing. When I tried to change back to Lancer, I got the error message, "Unable to equip set, items missing". I had to delete the armor set and re-equip each piece and save a new armor set.

World: Lamia
Character: Miiria Harvent