Depends on how they do it. Terraria has something called vanity slots, where you have two equip slots for a given piece of gear. One slot is for the look, the other is for the stats. Granted they solely do it for armor. A system like this has the same purpose of transmog, but achieves it differently as the piece of gear in question is not lost. I imagine from a programming standpoint that this is actually easier to do than Transmog, and can be added in post development far easier too.