I've had a look at a 'typical' ex-DUB I-class fares, and they appear to have a €300 change fee. So you that is the cheapest a change will be. If you can find I class on the flights you want to change to, then that will keep the cost down (probably to the change fee and any difference in TFCs). The good news is that changing the ticket after you have flown the outbound means that it should not lead to a reprice of the whole ticket.

If you have ExpertFlyer, do your research to find I class on the flights you want to change to (if available). Then ring BA and ask them to price up the cost of the change. As noted, it won't be less than €300.

As for the MFU, unless there is redemption space in F on the new MIA-LHR (or you used a GGL joker), you'll lose the upgrade, but I think you'll get the Avios back.