Thanks. I can hazard a guess as to what's happening here, which looks to me like a possible case of different sales offices seeing different availability because they're in different markets.

Your original base fare of MAD 250 looks like it may well have been an O class fare in both directions. (EF says MAD 245, which is tolerably close, and of course it may be different now from what it was when you originally bought it.)

Buying a new fare today at a base fare of MAD 1025 looks like it would book into N class in both directions. (EF says MAD 1120 for this, but again it's tolerably close and it is the next fare up.)

If you are being charged a fare difference of £81 or about MAD 1217, the new total base fare would be about MAD 250 + MAD 1217 = MAD 1467. This does not correspond to any single filed fare from BA, but is tolerably close to a ticket priced using N class in one direction and V class in the other (which is the next fare up). Using EF's figures, that would be (1120 + 1800) / 2 = MAD 1460.

Why is BA quoting you a mixed N/V itinerary when you could buy N/N in a new ticket? One possibility is that Moroccan market availability (which is what ba.com sells from) can see N in both directions. But if you're talking to BA in the UK, if UK market availability has no N in one direction but only V, then N/V is how your fare difference will be priced.

Having a look at the breakdown of the O class fare, I realise that I'd forgotten that BA has abolished the fuel surcharge on many short-haul routes, including (it seems) this one. That means that if you cancel the ticket, you might well get back everything except the base fare (MAD 250) plus the admin fee (which you say is MAD 300 for CMN) - so the cost of cancelling would be MAD 550. (But you'd need to check this.)

Without a market availability problem (if that's what it is), the two different routes give roughly the same result. Either you lose MAD 550 in cancelling, or you pay €50 or ~MAD 540 change fee. Either way, you'd buy up to the cheapest currently available fare, so that's the same in both cases.

However, the additional cost of changing the ticket caused by that presumed market availability problem is about MAD 350 or so, which would seem to give the cancel and rebook option the edge here - if (and that's a big if) my hypotheses are correct and the numbers are about right.