Have a look at the EC261 thread in the BA Forum Dashboard, if you look towards the end of that thread you will find quite a few other people in a similar position, and going forward I suspect we will see a lot of this due to equipment changes. Equipment changes could go the other direction too, so it's best not to be too fast on the trigger here, in case the First cabin gets reinstated. Generally though we are moving to a BA world with less First rather than more.

In that thread, go to the WIki and have a look at the Mennens formula, and this will give you some worked examples. EC261 is always claimed after travel, so the fare difference you will get at rebooking, EC261 is claimed after travel, not least because you may end up not travelling at all and therefore can't claim the reimbursement formula.