You should be able to. "Change" includes re-route, so I can't see any reason why you can't change the ticket for change fee plus fare difference. Then you simply have a WT+ ticket to your new destination, and I don't see why you can't MFU that. The changes will probably mean, though, that a lot of it will have to be done on the phone - but it does mean that you could probably do it all in one go.

You might want to have a look at Swiss Tony's experience in this thread: http://www.flyertalk.com/forum/briti...-bit-rant.html - he was actually un-MFUing on re-routing to take advantage of a good J fare, so it's not exactly the same; but it does suggest that there shouldn't be a problem with this.