I have a normal, i.e. not open jaw or mixed class, Amex 2-4-1 return flight booked.
Initially the change flight was not greyed out and I have been able to change the return date once on-line no problem.
However, after changing it the first time the change flight was greyed out and I approached Dilly for help.
He reported back a couple of weeks ago that there is a known problem with ba.com regarding this which won't be fixed for several months.
Therefore only way to change is to ring up and you shouldn't (!) be charged the change fee as it is impossible to change on-line. I haven't actually had to put this to the test yet...

Hope this helps.

Sue