My recommendation:

Do system rebuild on the current version, then upgrade.

You can use the steps in this TN for the rebuild: http://www.symantec.com/docs/TECH77447

Please note all requirements such as:
Hostname must remain the same
Install NBU to same drive letter/folder as before
Patch NBU to same version as before, then recover catalog.

Run on new hardware for a couple of days before upgrading to 7.5.

For 7.5 Upgrade preparation, see upgrade portal: http://www.symantec.com/docs/TECH74584

PS: For server hostname change, you will need assistance from a trained Symantec Consulting Partner.
The hostname is so deeply embedded in the different NBU databases that there is no DIY way of changing master server hostname.
See this TN: http://www.symantec.com/docs/TECH31385

Exctract from 2nd paragraph:

However there are many other operations that customers may need to perform at some point that require including migrating the NetBackup catalog information from one primary platform to another (UNIX to Windows or vice versa), rename a Master Server or Media Server, cluster an existing Master Server, or merge multiple NetBackup domains that should not be attempted without the assistance of a Symantec certified NetBackup consultant, either a Symantec consultant or a certified consultant from a Symantec Consulting Partner.