I couldn’t change mine as already on a pre-paid rate. Was hoping to trigger Amex offer butbpre/paidnrayes in US are coded differently and so this didn’t work.

To clarify a flexible rate can be cancelled free of charge up to a certain point before the stay. However, in the states HGI advance purchase rates can also be changed subject to a $25 fee(not sure if this is per night or per stay). Same rule applies for other Hilton group properties although fee increases to $50 for Hilton etc. Must book another AP rate when changing and has to be done by a specialist team.