Facts

Currently in WT
At time of booking, cost in WT+ plus either 125 or 150 additional payment each round trip ie either 250 or 300 total
Unable to price upgrade online myself in MMB with a message to call BA directly
Called BA directly and received a price difference of 100 change fee, 25 service fee waived as I am unable to self upgrade on website, fare difference of around 225 and credit with change in taxes from original booking for return upgrade both sectors journey
Back office by email priced the same change differently by a magnitude of approximately 100
Fare rules as I understand require the fare difference to be based on available tickets at the lowest price in the upgraded cabin which in fact are available in T class (same class that was available when I first purchased tickets) such that the fare difference should be 150 of 125 + 100 + taxes adjustment either increase or decrease

Query

Is this a case of HUACA? given different pricing
I would tend to go with back office manual pricing versus telephone agent pricing?

Or have I misunderstood how this works?