I have kept an eye on the prices for upgrades and can confirm they vary wildly. For example, an upgrade to WT+ LHR-SFO previously cost over £900, now suddenly there is a special offer to upgrade for £104. I guess that mean they could not sell those seats at the full price so trying to fill them up at a bargain price...fair enough.

The cost of upgrading on my return, SFO-LHR also fluctuated, it started at something like £1500 for an upgrade to CW...now it's at over £3000.

So I guess the moral of the story here is that you can either upgrade as soon as possible, and have a guaranteed seat in that cabin, or you can gamble and see if the prices come down - which they may or may not.