Votes on the motion to proceed with S.649 are a much different thing than votes for specific amendments or the actual base bill. However, I'd guess the fix has been in for a while on the sudden "deal."
Toomey announces his amendment tomorrow, S.649 goes to the floor on Thursday before the proposed amendment can even hit Thomas. That is probably not an accident. They are trying to minimize the amount of time people have to read the amendment. I imagine Toomey didn't come alone to this party. McCain will jump onboard. Coburn already supports UBC with some caveats. Two more Senators (Ayotte or Collins?) and you've got 51 if no Dems break ranks.