I have done this several times with my Hilton, IHG, or Marriott certs in US or Asia. Contacted hotel ahead of time saying I am booking a free night for a room at current rate x for my date can I pay the difference in cash to better room’s rate y?

Of the 6 or so times I have asked, have received a yes. Though I usually only have 1~2 certs at a time, so these are short stays. I could see length of stay being a factor.

As suggested contact the hotel. And probably better to contact before having a reservation - i.e. a question about a potential stay - implying that their answer impacts whether you will stay there or not.