This question has been asked in many threads.
Today I can answer it.
Yes they do!
I have just checked upgrading an upcoming return leg from CW to F. The price includes a reduction in the fuel surcharge of $95. After the increase in taxes and fees, this would leave me a credit of $46.60.
Unfortuately, I won't be taking advantage of this windfall because BA want $29554.40 to upgrade LHR - BOS, CW - F!

