Changing the lead traveller could be a challenge but you should be able to change the accompanying traveller for a nominal fee. Lead traveller changes aren't impossible but the policy varies with some companies demanding a full cancellation (with loss of deposit) and rebooking, whereas others are a bit more accommodating. No idea what the BAH policy is, sorry. I expect both you and your ex would still like to take a break so it's well worth calling up to ask what the options are.

I'd personally look at name changes and take a friend. I've had many an impromptu holiday due to a friend who booked holidays and subsequently split up with the person they were dating at the time of booking. They've settled down now!