Are the conditions on the CW fare that you are looking at more restrictive than the WT+ fare that you are upgrading? Are the cheaper WT+ fares already sold out? Have you called to upgrade just the longhaul segment?Have a look at the discussion here for example:
http://www.flyertalk.com/forum/briti...os-ripoff.html
http://www.flyertalk.com/forum/briti...expensive.html