Yep Natty, it's much easier to just stop showing a dog that is on main register than it is to buy a pet on limited and upgrade it.

Both my poodles are on main register, one only went to 3 shows and the other was never shown at all (he blew to height restrictions for the breed) So they are just happy house pets.
You don't have to do anything to stop showing... just don't bother renewing your membership.