If it were my bike I'd "upgrade" to 8-speed and bar-end shifters. Or just keep what you have until it fails. Your crank and derailers should work fine with whatever bar-end shifters you decide to use, as long as the chain and cassette match the number of speeds as the shifters.
Originally Posted by
ricohman
After seeing the new Trek 920 I realized the Sherpa has clearance for at least 2 inch tires and I would like to make it trail worthy.
Are you sure about that? It doesn't look like it to me, but I'm just guessing based on the photos.