I have booked a 300k 7 night stay on points at Ritz Carlton Tenerife over New Year. I didn’t have the points when booking so made a reservation with the proviso of obtaining an e-certificate at a later date.
I am now thinking of going for a 7 night air hotel package instead. I should have enough points by late November. However by that time the bonus period for the 35% extra avios will be over.
What is the chance that I can book a 7 night hotel air package Marriott Cat 1-5 now then come November phone up Marriott and ask to upgrade that to a Tier 1-3 Rtiz Carlton air hotel package for my Tenerife stay thus at the same time applying the e-certificate to the 300k provisional points booking I currently hold.

Any advice is appreciated as I want to be certain before I do it as there is no longer any points availability for Tenerife!

I'm not sure I entirely follow what you're asking -- but booking a hotel/air package and asking to upgrade or downgrade it at a later date is possible, and numerous people here have reported success in doing so.

Ok, I want to upgrade but also use it to replace my existing points booking (for which I'm yet to earn the points)

yes, you can book a CAT 1-5 TP now and 8 days (or something along that line) before your check-in date of the hotel, call into Marriott Rewards (not Reservation) and ask to upgrade the CAT 1-5 cert to the level that you need and attached to the reservation.

I only redeem CAT 1-5 TP and upgrade when I need to.

As alc mentions, upgrading a TP hotel cert from a lower Cat level to a higher one shouldn't be any issue at all. Lots of posters have done it (my current TP hotel cert sitting in my account right now was originally a Cat. 5, but I upgraded it to a Cat. 8.)

But I don't remember seeing any posts (though there may well be some) of someone upgrading from a Cat level to a Tier level hotel cert. My guess is it shouldn't be a problem, but not completely sure. I'd suggest calling MR and asking if they would do that. While one agent saying they will do something doesn't mean that all of them would, it would be a fairly good indicator that there shouldn't be an issue.

Hopefully, someone whose attempted it will chime in and let us know.

Ok, thanks. I will phone twice and ask the same question. Providing there is the mechanism for doing what I propose then that will be fine by me.

On email they said, yes no problem. When I phoned both the UK and the US number though both agents said "NO"!