Typically when you change the outbound leg, you have to re-price the entire itinerary as if you were making an entirely new booking. This may be the case with full fares as well, although in that case the new fare shouldn’t be much different from the old fare. So there’s likely something going on that’s resulting in a 4k fare difference. For example if the original ticket had a 90 day advance purchase requirement and now your new outbound is only 2 weeks away. Full J fares usually don’t have advance purchase requirements so it’s unlikely you actually had one. F course you may have a refundable fare, those can often be had for only a few hundred £ more than the cheapest fare.