Any retro change of billing is considered cancellation of the billing and re-billing. The only way to do it is to cancel early and re-book it. It is close to impossible after the booking has been charged. For prepaid hotel like Expedia, the hotel has nothing to do with it. You'll have to deal with the travel agent, which is a bigger pain. This is one of the reasons travel agents are not recommended since you do not get rewards of such stays.

I recently made a similar mistake of using a CC that I thought I would get discount. But I could not (only valid in US). So I just let it go without going through the trouble.