by Ken Task.
Don't know for sure, but wonder if this might not help ...
Make a full backup of one of the two courses - including users. Download the backup. The backup file from a 2.6 is a zip - just has a .mbz extension (unless you've turned on experimental items related to backups). Unzip the backup. There should be a users.xml file among the extracted folders/files from the backup. Open that with a text editor and see if there are references to cohorts.
If there is, then one could restore courses to the 2.9 .... just to get the users and cohorts established (maybe).
'spirit of sharing', Ken