This also depends on how much you've already paid in TFC (particularly YQ).

On current fares, if you originally buy a DIF that books into T class for travel in WT+, the YQ is £149.

If you buy a non-DIF T class fare, the YQ is £250, and the same applies to E class and W class fares.

So if you UuA from a DIF, you'll be in for £251 in additional YQ (to reach the CW level of £400), together with any other differences in TFC (typically because of exchange rate differences or increases in tax levels since originally booking, eg the recently-introduced £8.90 R1 charge at LHR).

But if you UuA from a non-DIF T class fare, the additional YQ is only £150.