Create a new Premium Shop item "Scroll of Reforge Rarity Protection."
That way, if you have a [Legendary] or [Epic] weapon, and you still want to try to upgrade it to an [Ancient], you can still reroll the rarity without losing the current rarity it has.
That should incentivize some Logic Point purchases and help people maintain their equipment without the risk of rerolling a Legendary back to an [Uncommon].