From memory I don't think the slide portion can be removed
I re-U channeled my Drivers side 3-4 months ago. This is how I did it.
With the window closed, pull the U channel from the front lower corner until I got to the sliding window. Cut the old channel off. CAREFULLY slide the window into the slot you just emptied. Pull the remaining U channel off back up to the back edge of the now open window. Cut this off too. CAREFULLY slide the window back toward the closed position. Remove the top front section of U Channel. I cut a section that was longer than the piece I just removed. This is going to create a seam, I planned it to be at the middle of the top of the sliding portion when closed. Slide the end over the top of the closed window until you can get the other end inserted into the track.
Slide the window into the open position again. Then using a section longer than necessary slip the new U channel under the bottom of the window from back to front. The U channel will fit, but you will have to work it some. You may have to lay the U channel in the groove part way back and slide the window glass on it, then pull it through with a pair of needle nose pliers, or work it using the sliding back and forth of the glass. Your goal is to have a single piece from lower front up to the seam at the top.