I agree with Jonathan - simply change policies to start using new Storage Unit.
The only decommissioning steps necessary will be to change ownership of media belonging to master:
bpmedia -movedb -allvolumes -newserver <hostname> -oldserver <hostname>
Followed by deleting devices attached to master and Storage Unit.
You can even choose to keep one tape drive attached to Master so that it can back up its own catalogs locally.
In that case, Master server will still be available to restore backups that was done by it up to now - so no need to change media ownership. Just change Storage Unit properties to 1 drive.
Whichever choice you make - no binaries are de-installed on the master. Just change configuration.