Exactly, that is what I did, made a TP registration for arrival 29 April. After getting the approval I changed tickets to arrival on May 1 and cancelled the SHA+ hotel. TP is valid for 7 days before and after the registered arrival.
After my application for a COE was approved last year, the quarantine was reduced from 7 days to just one (as well as a change to TP). I simply cancelled the 7 day quarantine booking and made a one night booking with the same hotel. That was accepted and I expect that for the change on May 1, the same applies. The rule change is for arrival from May 1 onwards. In the past reductions in quarantine period have been linked to the date of arrival, not when the registration was made.