I was able to use the Kaer Morhen Armor to craft a Warrior's Leather Jacket, and I'm wondering if any other pieces of equipment that I would otherwise sell or disassemble could serve a similar purpose or be useful in other ways.
I think it is just the various types of Witcher gear that you find in treasure hunts that can be upgraded.
These are the Feline, Griffin and Ursine sets (plus Wolven added by a small free DLC).
These can be upgraded three times (four with the Blood and Wine paid DLC) known as Enhanced, Superior, Mastercrafted, and Grandmaster. If you play through again on New Game + the names are prefixed with Legendary.