Does anyone have any experience where BA.com and the call centre have different fare differences ? I have checked everything else, fare buckets still available and combinable and trying to reprice with current fares.
Did you check the fare availability from the POS (Point Of Sale) of the original ticket?
As above, you need to provide much more information including the fare basis you were originally booked on, full routing, POS, fare rules and what you want to change to.
How did you price it up and come up with 1628 CHF, and why do you think you can change to this fare? I would guess that the fare you are trying to change to with a difference of ~608CHF is not valid to change to from what you are booked on.