I've had conflicting results with this - sometimes I've been able to change segment after flying, sometimes not. This has been on both award/cash flights, as well as alaska/partners.

I have even seen at times that for the same flight reservation I'm able to make 1 change (after flying), but trying to do a second change shows a web error.

So ... not sure how exactly the rules have been coded, but its sometimes possible, sometimes not ¯\_(ツ)_/¯