The introduction of the SIVA armour being rerollable was a solid intro to the idea of armour perk rerolling, which is far less aggressive than rerolling perk weapons in terms of how drastically it can change the effectiveness of the item. The cost to 'reforge' the armour (no reforging weapons) should reflect the rarity of the armour piece in question. Perhaps special metal insignias from Iron Banner to reforge the armour, and a stack of Trials coins for reforging that set. Maybe you could even turn in the armour piece and a large stack of the reforge item to get a different piece of armour entirely? Such as a helmet and 20 Trials coins to get a pair of gauntlets.