I have my standard answer for this! It has been deployed for all similar UuA queries recently!

There is an extra cash charge going from WT+ to CW and also WT to WT+, but no charge from CW to F. In all cases there is no change fee to pay.

For a UuA from WT+ to CW you should be paying the difference in carrier surcharge - it isn't a tax difference. The carrier surcharge applicable is route specific, generally though the longer the route the more the surcharge. 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, and 10k avios and £53 for a one way UuA.

For a UuA from CW to F there is no difference in carrier surchage or APD, so it will just cost you the avios required.

Y to WT+ UuA isn't very common, but it is worth mentioning 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. As the APD is payable for leaving the UK and not arriving, the extra APD would only be charged if you did a UuA from Y to WT+ on the outbound from the UK. If you just UuA the inbound from abroad back to the UK there is no extra APD to pay.

Also, in all cases any changes in taxes, charges for customs, immigration, YQ, airport service fee, passenger service charge which have happened since the original booking was made will be taken in to account.

So, one way UuA from WT+ to CW for LHR-HKG should be avios and additional carrier surcharge (around £65 from memory for that route), give or take a few pounds for possible other fee/taxes changes.