OP:

I tried the very same thing 2 weeks ago. Was told that up-faring does incur the change fee, although up-faring to a different class does not.

A week later I called again and asked another rep: he was able to grab me an M fare (had T initially) and use my SWU, no change fee.

I believe the official policy is that they do charge change fees, but some agents are willing to waive that (which I believe makes sense).