This must be very frustrating for you. Unfortunately, the flexibility you're seeking now is precisely what you gave up when you bought the I fare.

You'll need to check the terms and conditions for your fare. It is possible that BA could choose to reprice your outward trip as a single (which, as you've seen, could make it much more expensive tha the discount return you purchased). Others will be better able to say whether they would in fact be likely to go that far.

A tiny glimmer of hope (but really very small) is that sometimes, just sometimes, ba.com is kinder than it probably should be. I've been surprised by lower than expected online quotes for itinerary changes. But please don't raise your hopes.