You could recreate all policies. We are currently developing a solution to migrate all configuration from one Symantec DLP to another (change mangement staging) and I can tell, it is not easy.
But after you have re-created the policies, you could fire all original that the previous tool found through Symantec DLP again. this would recreated the incidents again. Unfortunately they would have other date etc. but that might be a minor issue that can be addressed in the notes.
Rescanning will be easy for DaR, not so easy for DiM Mail (you would have to recreate the mails first), impossible for Network Monitor or DiM Web and impossible to tell for DiU (depending on endpoint profile).
Based on the configuration change tool I mentioned, we could theoreticallcy migrate one tool to another but we would have to understand and adapt the other tool first, and write a migration mapping. This is by far to much effort unless customers really uses DLP as incident management for long running legal cases. But even then, exporting them to another tool or even PDF, should do and is much cheaper.