I must admit I have never tried this, but I have an upcoming ticket (on BA) for which I need to change the return, as I need to return home sooner than intended when I made the booking (during a ridiculously cheap sale, so absolutely no scope to change the return date prior to departure).

I know the fare rules say I should be able to change for the historical fare after departure, but I am also a bit paranoid that the availability will not be there, or that some other fare rule or restriction will kick in to nix the ability to change the date to one I find more favourable.

Just in case, I've already locked in a Flying Blue award to get me home, just in case it all goes wrong and I have to forfeit my BA return due to being unable to change it to an acceptable date at an acceptable price.