You can buy a Premiere at Best Buy and when you go to activate it on tivo.com, they will offer you $400 lifetime.
However, your $6.95 Premiere is tied to your S3 box. So, if you sell your S3 box, you no longer have a full priced unit to qualify for the $6.95 rate and your original Premiere will revert to a higher monthly price (i.e. $15, but maybe they'd give you the current MSD price of $13).
If you opt to pay full price ($500) for lifetime on the new Premiere, then maybe they'd let you keep the $6.95 rate on your original Premiere, if you sell your S3.