Eleven SPL sides must approve the proposals to send the plans to the Scottish Football League later in the week.
Both leagues and the Scottish Football Association announced agreement in principle on 8 January, with the 12 SPL clubs apparently achieving rare consensus.
But since then, St Mirren have indicated they will vote against the proposed 12, 12, 18 set up, while Ross County have still to make their intention public.
Celtic are the latest of the 10 clubs to back the plan, and chief executive Peter Lawwell reckons his Ross County counterpart Roy MacGregor may regret any decision to go against a new set-up.
Lawwell said: "Maybe in two or three years' time, when the novelty factor has worn off and the quality has kept coming down, Roy might say 'that was an opportunity lost'.
"Maybe he's right, but 10 of us don't agree and we've been around for years. It's not a threat. It's not an ultimatum.
"It's just trying to get him to understand what we know and he has no reason to know because it's new to him."