I don't think there will be a grandfather clause, in the sense of being able to sign players in November with absolute impunity.
IF (big if) there is a cap, I think it will include one or both of the following:
* A phase-in period. Let's say they agree on a cap of $280M. Maybe the phase in period is three years. Year 1 - $340M. Year 2 - $310M. Year 3 - $280M. (With annual escalators.)
* A one-time amnesty provision. The NBA has done this. They impose the full cap immediately, but allows teams to get under the cap by cutting players. The team still has to pay these players of course, but they don't count under the cap. If this were to happen, the Mets would likely cut Semien, Manaea, and Polanco. Maybe also Senga and/or Williams. Problem solved.