Ah, a rebooking is not always the same as a rerouting. Normally when rerouting the point of origin should be the same. I think this is fairly standard airline practice, but as I've been out of the ticketing environment for a few years, I can't say this with a 100% assurance that things havn't changed.
LCC pricing models are different too. Squeezy might permit this type of change but ...
These things are hard-coded into the airline's booking systems. Over the years airlines have lost significant sums from currency manipulations so they have programmed their systems to avoid such losses. Obviously they didn't intend to restrict a EUR/GBP transaction but better safe than sorry. Call KM again and ask them to see if they can't do a refund against a new issue. If they can override the NONREF tag, they should be able to apply the value of the original ticket against the value of a new ticket.
Good luck ...