How to make upgrade for e-Cert reservation?
 

I received few free night worldwide certificates, but I can't book anything in Europe, particularly in UK. We have 2 adults and 2 children, but all hotels there can accommodate only 2+0 or 2+1 in standard rooms and this certificates are good only for standard rooms. But they also have family rooms good for 4 people.
We could get a complimentary upgrade as Gold members, but it's subject to availability = not guaranteed.
We are ready to pay for upgrade, but how exactly to do that? When I select a regular cash rate - it shows few paid upgrade options, but when I switch to e-Cert - all upgrade options disappear... :(

If possible at all only through member services or the hotel reservation line. I would probably book with the certs online and then call the hotel to check if they are willing to confirm an upgrade for some cash supplement