Is the new fare a sale fare whereas the original booked was a normal fare ?
Or, it may be 2 fares which are not combinable. In this case, however, you may find it cheaper to change both sectors to business than just the return.
Fare rules and calculation are extremely complex and while I know BA Bremen often gets it wrong, don't assume that they always are. Try calling again and see if a different agent can either be more imaginative or at least explain why it's so expensive to change.