Hi Joel, Done this many times---Mark where the original channel is on the glass. The remainder of the window channel has to be pried off--use plastic or wood tools. If the "rubber" piece is original the channel will be pinching the rubber/glass very tightly. Easiest way would be to get the channel spread along its length with a wedge tool----and then inch the channel off from the bottom edge. The metal should come away from the rubber. Then scrape off the rubber with a razor blade.
An assistant is helpful in all of this--especially the going back together part. Work on a steady wood surface. If the other poster had his glass "glued" into place he may just be lucky that it hasn't come apart. The channel is meant to grip by squeezing. The original "rubber" from the dealer works best because it resists water--but I've even used sheets of newspaper to cushion the channel to glass as a temporary measure.
Place the glass on the work surface bottom edge up. Lay the rubber down folding it over the glass. The assistant should hold the glass and channel steady--feed the channel at a slight angle and use a plastic mallet or block of wood against the channel and a hefty hammer (but not too hefty to control) to strike the blows. Get one corner of the channel started and work your way down the length till the entire channel is started. Now you should be able to knock the channel down to its full depth from a couple of spots. Trim the excess rubber--or not.
The whole thing is a bit scary. I know you'll be fine with it (call me if you need to)--but I do have glass you can have should you need it---will be visiting in Winston-Salem at the end of April. -- Dave