If you want to change your Avios flight booking to Avios and Cash (not UuA, just Avios and Cash), can you call the call centre to have it changed, or should I cancel and rebook?

There are plenty of seats so the availability isn't an issue. But I'm wondering if I can just call up and have it adjusted?
I have my flight recorded on my computer already, and I don't want to have to update my records with the new PNR and all that (serious laziness, I know).

The reason I'm thinking of doing this is the horrifying rate I have suddenly decimated my Avios balance - so much so that I am now uncomfortable and want to get some back by changing some bookings to Avios and Cash