To do this, you can probably just get their HumanoidDescription, serialize it, and then save it in a data store to later load it in.
When you eventually want to load in theit avatar, you can just load in the data, deserialize it, put all that data in a blank HumanoidDescription, then apply it onto an NPC.
I don’t think you can get someones old avatar aside from getting their saved outfits (which you can probably get via HttpService)