Mailgirl4now - are you saying the H route is becoming vacant and needs someone to carry it every day for some amount of time?
If you are the only RCA in the office, you can become the primary on the H route so you can fill the vacancy. A newly-hired RCA would then become primary on the K route. Or, you could retain the K route as your primary, and still carry the vacant H route, since you are the only one available to do it. The newly-hired RCA would then be hired as the primary on the H route, and will carry it until it is no longer vacant. You would go back to subbing on your K route then. Either way, you retain your seniority for bidding when the time comes.
Personally, I would keep the K route as my primary. It is nearly impossible to hire a new RCA for an H route once they find out they'll never get to work. If you switch your primary to the H route, a new RCA is much more likely to be hired. Then you'd lose out on working every week once the H route is no longer vacant (unless of course, it's going up for bid, and you become the next regular).
If I'm wrong about the H route becoming vacant, maybe you're just looking to retain your seniority for bidding but don't want to work every weekend anymore. In that case, you can switch your primary to the H route, but you would still be subbing on the K route until a new RCA can be hired and trained.