The MWG should automatically upgrade the policy for you, if it doesn't, then there's probably something in the configuration which is breaking the policy upgrade. There may be hints in the mwg-core or mwg-coordinator logs.
Please open a support case if you haven't already and we can help identify the problem to get the policy upgraded.
Did you try to upgrade the 7.3 policy to 7.5 or did you try to export the 7.3 policy and import it to a 7.5 system?
If direct upgrade, that should theoretically work.
If export of 7.3 and import to 7.5, I wouldn't be surprised if that breaks things.
We recently upgraded 7.2 systems to 7.5.1 and and due to issues with direct upgrade, I punted and rebuilt the rule set from the ground up. No regrets there, although it took some time on the front-end of the process.