Hi Chris,
This is not a supported workflow, but there are no known issue with a 10.2 client application connecting to a 10.3 Workflow Manager database. We recommend that users upgrade everything (Desktop, Servers and Database) to the same version together, this could pose a challenge in organizations with many users that could prevent the upgrade from happing at the same time.
So here is my recommendation - upgrade your database using the version post install that you want to eventually upgrade the applications to and then upgrade your WMX application as soon as possible so that your users can start taking advantage of new capabilities.
I hope this helps!
Thanks,
Tope