The site ID is the Support ID under which you purchased the licenses. So you will need to work with your BMC Sales representative to determine if you are entitled to change your Support ID ( for example, if one company was merged with another).
If you are entitled to do so, and are also changing the server upon which the license is loaded, then yes - you would need to supply the hostid of the new machine and purge the license from the old machine.
More information can be found here: Licensing and Passwords
Note, however, that you are on an unsupported version of the product. Rather than transferring the license, it would be the best recommendation to upgrade to a later version of the product so that you are running on a fully supported product. Your maintenance contract will allow you access to the latest versions on the Electronic Product Distribution (EPD) site.
If you are not under maintenance, you may not be able to request any of the above functions. Again, contact your BMC Sales representative to determine what your next steps could be.
David J. Easter
Product Line Manager
BMC Software, Inc.