Liberty is totally right on the requirement that work done on a beltloop prior to being a Webelos doesn't count if the boy is working towards earning the beltloop for a Webelos Activity Badge requirement.

But the beltloop he may have already earned doesn't expire. He still earned it and he still wears it proudly on his uniform. I think that was the point most were getting at. Once a beltloop has been earned and awarded, it remains on the belt the rest of the time the boy is a Cub Scout – unless he wants to remove it on his own. But nothing requires him to take it off, and while it can (and in some cases for Webelos) must be re-earned, the one he earned first never expires.