What are the proper steps to decommission Symantec Backup Exec 12 Ver 12.0 Rev 1364?

Current setup: Server A + LTO3 autoloader + Symantec Backup Exec 12 Ver 12.0 Rev 1364 + multiple agents.  Have 5 sets of LTO3 tapes with backped up data.

We want to decommission this setup and bring in a new setup.  The new setup will be "Server B + LTO5 autoloader + Symantec Backup Exec 2012".

1. What are the proper steups to decomission BackupExec 12 and the old agents?
2. What steps do we take to ensure the new setup can read and retreive data from the LTO3 tapes?

Any assistance will be appreciated..thanks! :)