Hi headingwest. Yes there is an extra cash charge going from WT+ to CW, but I am fairly sure no charge from CW to F.

For a UuA from WT+ to CW you should be paying the difference in carrier surcharge - it isn't a tax difference. For example starting from the UK the additional surcharge for CW over WT+ for a return to NYC is around £106 so the cost of doing a UuA for a return should be 20k avios and £106.

From Y to WT+ there isn't any additional carrier surchage. There may be additional APD in some cases though. For example, the APD for a Y class return from LON-NYC is £69, whereas the APD for a WT+ class return on the same flights is £138. This is actually the extra APD due for upgrading the outbound from the UK. If you just UuA the inbound from the US back to the UK there is no extra APD to pay. If there is extra APD due for the Y to W UuA I would expect BA to charge it when processing the upgrade.