This has not happened to me, but it should return to you as the sender , as undeliverable. In my opinion it is the buyers responsibility to keep her account profiles {in this case paypal} up to date. I wouldnt concider this your responsibility. I would write to the buyer and let them know that when the package comes back to you, you would gladly ship it to them at the correct address for the shipping fee.
That of course is my opinion and some might go another route on this one to keep from any possible negative feedback.
Hope it all works out.
Tink~