Go to the main support page at
http://www.symantec.com/business/support/overview.jsp?pid=15047
On the right, select "Compatibility List" for each version to compare the two versions.
Just on GP, I'd upgrade the v9.0 to the v9.1 with the latest SP. That was one of the most stable builds/versions veritas ever released, and it's free
Additionally, v9.x is officiallly End-Of-Life, so while Symantec will offer to help if you call in a problem, the will not do any troubleshooting on their end.
Also, V11d is two releases old, so if you do upgrade, I'd recommend v12.5 (BackupExec 2010 is due out early next yeat