Correct, there would be no automatic pay cut when single carrier status is established. However, the finding of single carrier status would trigger a vote by the combined pilot group on whether they wished to be represented by the IBT1108, not represented at all, or represented by another entity. Exactly how that vote would be structured, i have no idea.
As I understand it, if the vote is in favor of representation by the IBT1108, the combined group would be covered by the existing Flight Options CBA, since the Flex crews have no existing collective bargaining CBA, which would result in all crewmembers reverting to the FO pay scale unless the CBA is somehow amended by LOA or whatever.
I'm not a labor lawyer, this is simply what I have learned from watching the process, researching, and speaking to people involved in the process.