I might be wrong but I suspect the ICC will insist that you cancel and re-book as your originating city is changing.
If you can, I would make this as a seperate booking and once that is confirmed, cancel the first. It will cost you £5 more in cancellation fee (instead of the change fee), but you can be sure you still have a flight.
Note that if you cancel your flight there is no certainty the seats will become available again - it is a gamble!