Hello,
Any backup that is used to restore a server must have been generated on the same version of software as which it will be applied. For example, a backup of VSOM or VSMS server created on 4.2.1/6.2.1 can only be restored on a server running 4.2.1/6.2.1. You cannot restore a 4.2.1/6.2.1 backup on a server running 6.3.2. Therefore, if you were upgrade from 4.2.1/6.2.1 to 6.3.2 and the upgrade fails, then you would need to downgrade the server(s) back to 4.2.1/6.2.1 in order to restore your servers using the 4.2.1/6.2.1 backups.
You can find the 4.2.1 VSOM document and the 6.2.1 VSMS document on our website, and they both include the backup and restore process for their respective processes.
Also, if you are going to upgrade from 4.2.1/6.2.1 to 6.3.2 you many want to use the Cisco VSM Software Assistant 6.3.2.
Hope this helps,
Sean