Is it possible for a pst rollover threshold change take effect on a migration that is in progress?
It depends if the data is imported into a PST file or not:
If data hasn’t yet been imported into a PST in the staging area (the temporary PST which is created during the export/import, aka Stage 1) then YES. You can find the MailNativeContainerID and update the threshold in the MailNativeContainer table. Or it can be done on the Existing Mapping screen as well.
If data has been ingested then NO. It would need to be processed as a mapping delete/recreate.