Where prepaid rates are not charged by the hotel (US, Mexico and Caribbean), the reservation can be changed for a $25 or $50 fee, provided you contact Hilton at least 3 days beforehand. The property can be changed to any other one where prepaid rates are charged centrally. The new booking must also be a prepaid rate which you pay for first, and the old booking is then refunded minus the fee.
Thank you I did not know this. I guess though if it is a hotel in say the UK this does not apply.
In general regarding the OP, I agree it would be a case of asking if the hotel would allow this. I have become more nervous about booking non-cancellable rates recently as all of my travel is by public transport and thus my plans may be changed for me!!! I kind of see the full rate, cancellable day before, as a hedge or an insurance cost.