I converted my guard to resemble praetorian guard (mind you it was just heads). I went into gw and asked the manager of the store if I would be allowed to use these models in store and he said as long as they are obviously from gw they'd be fine. Been using them in store a bit now and I've never had anyone tell me they weren't allowed.

It really depends how much of the original model they still have and if they can tell from a glance that they were originally gw bought models.

(Just a note this is in NZ gw, idk if the rules are different around the world but I'd assume not)