My advice is to just keep trying. Different reps seem to have different levels of knowledge and ability to achieve these changes. It’s also possible that their IT is allowing certain types of changes/bookings only sporadically...

I had phoned several times already over the last week trying to convert a cash booking for St Regis Bali into a points stay, and was repeatedly told that it could not be done (for a variety of reasons).

But tonight I was finally able to convert a St Regis Bali cash stay into a points stay. 11 nights in April for an average of 53k points per night. Quite pleased, although hoping this isn’t too many nights in a row at this property...