You can probably put a set of 650c wheels on the LHT, depending on how much adjustability you have in the height of the brake pads. But every time you switch from 26 to 650c, you'll have to adjust the brake pads up or down. If you have canti brakes, this might be more than you want to deal with. Alternatively, you could run 26" rims with narrower tires, but 26" rims don't come as narrow or light was 650s. There's a chart on Sheldon Brown's site that will allow you to judge how narrow you can go with tires based on the width of your rims.