Ambassador, British Airways; FlyerTalk Posting Legend
Join Date: Apr 2012
Location: Leeds, UK
Programs: BA GGL/CCR, GfL, HH Diamond
Posts: 42,881
Is it a voluntary change or an involuntary one?
For the former, yes if you change expect to pay the avios difference. It may allow you to change online without the extra, but if you call you will be given the official policy.
If it is an involuntary change (so the result of a cancellation) you should be able to change up to end of June without any charge and without the need for reward availability.