Junior,
The 6 and 7.5 MUD's are not interchangeable. The v6 MUD will not unlock PC-Doctor 7.5.
It may be less expensive for you to upgrade your version 6 to version 7.5. This is normally accomplished by sending in the v6 dongle and receiving a v7.5 replacement dongle and new media.
Please contact sales to find out your best option. 775-336-4000
-Fred