+1

Do this before rebuilding the indexes - IIRC, checksums are only written when the data/index page is updated.

Rebuilding the indexes should ensure checksums are created for all your data.

I'd recommend setting Compressed Backups as the default & switch on 'Optimise for Ad Hoc Workloads' option, but they're not really upgrade considerations 🙂

Have you run the Upgrade Advisor to check your SQL is compat-90 valid? Or is that the application vendor's problem?

Cheers