A lot of the above can be explained by timing, and how fares get calculated.

If you get reticketed into a lower cabin, then the ticketing agent will charge the appropriate fare and Avios, and with that you merely get a straight fare difference and the Avios get put back into your account. Once ticketed in the lower cabin, then if for some reason the cabin is offered again, then the ticket will have to be reissued with the higher number of Avios and that difference would be deducted from the Avios account. All of this, evidently, is done before travel. Ticketing doesn't handle the EC261 components.

In terms of EC261, that is a Customer Relations task, and there are different reports on how BA handles the Mennens Formula (see the EC261 wiki). But CR will only look at this after travel, since it's only at that point the downgrade would have happened. With equipment changing so frequently it would be best to wait to see what actually transpires. At that point the Mennens Formula applies - and CEDR will uphold that if necessary - but BA are entitled to take into account the previous fare difference refund.