For the past few years CO has waived the change fee if you are switching to a bucket that gets you into the forward cabin. This would include switching to a B fare to use the B-Up feature that the program offers.

It is a bit less clear with the change to a W fare as technically you're not moving to the forward cabin directly but my guess is that you could make the claim to the company that it is the same and get a cooperative agent to waive it for you.