You won't be able to install any fixes from BAW to BPM 8.5
Thus, you have two options:
a) migrate TEST
b) do you full system test. There is a good chance that your BPM app won't work as is and some changes have to be made. Depending on your BPM 8.5 version, you can deploy snapshots from BPM PC to BAW PS. You have to check with support on exact versions.
c) once you fixed all the errors ( I know it would be a pain to deploy snapshot to PS for each test case ), you can upgrade you PROD.
d) upgrade your PC last.
As an alternative, this is what I usually recommend, you can install a NEW BAW PC.
Perform artifact migration and use it as your "new" PC or a temporary one.
This way you would have your OLD PC and NEW PC running in parallel. It is covered by IBM licensing for 3-6 months during migration. Check with IBM rep for more details.