Have you been asked for GBP 300 as change fees or as fare difference?
Sounds like EK is asking for a fare difference due to existence of multiple fare buckets within the Flex range. There are multiple fare buckets (W, Y etc) all of which are flex and you may have booked into the lowest flex fare which may not be available for the flight you want to change to.
The issue is worse if you have two sectors since both sectors have to be issued in the same fare bucket IMHO
Some ticketing guru here might be able to shed more light on this . . .