If you are just using it for DIRT, then its no problem.
However... I would probably do the following:
Install Unity 4.05 on the new server
DIRT Export the old server
DIRT import to the new server
In place upgrade to 7.x
This way, if you have any problems, your production server is still running and easy rollback plan. If you upgrade your production and something happens... you cant rollback. (could if you pull a drive)
Unity will still start and run, but the Subscribers or ports will not register when the license is not there. Which is fine for DIRT, it does not care. Its only accessing the database to pull out what it needs. (everything)
Be prepared for licensing issues. They have been extremely slow lately