Upgrading to BE 2014

I will be upgrading from Windows 2008 R2 to Windows 2012 R2 and from Backup Exec 2012 to Backup Exec 2014. There are many backup sets in my BE database. I want to make sure that they are not lost during the upgrade. So:

1. Should I upgrade BE before the OS or the other way around?

2. Is there are literature that Symantec has published that I should read before doing this upgrade?

Thank you for your help...