I might be getting mxed up here, but won't the Opticron zoom screw onto the threads around the Kowa's bayonet mount-I seem to remember reading somewhere that it did (or perhaps that is a different Opticron HDF zoom?).
I have no doubt that the Kowa will last. I had my TSN-3 for 15 years and there was nothing wrong with it (until an ambulance ran into the back of my car and wrote off both-the car and the scope!) In comparison with modern scopes, the design is a little dated, and the Kowa isn't waterproof, but in all those 15 years mine never steamed up once.
Personally I would never sell to a shop. They'll offer you next to nothing, and sell it on for top dollar? TSN-1's still sell regularly on Ebay for around £200-give or take, depending on condition.
If I were you, and I wanted to see a significant improvement in image quality above 30x, I'd consider trading both scopes in for a good 80mm ED scope, but don't let me put ideas in your head...
Steve
ps. If you're not a confident ebayer, try posting an ad in the classifieds section of BF. It costs nothing, or you could also put in a request for a used Kowa wide-angle. Some of the dealers are also worth checking for used equipment-try the websites of Birdnet Optics, Cley Spy, In Focus, Kay optical etc.