I never tried to make any changes to the first segment. The segments I was trying to change were in fact the last few, involving HEL and CDG, and the first part of the ticket had already been flown.

The ticket picked out above by jerry a. laska is exactly the one I'm talking about. It was issued by AA with only one small hitch, described explicitly there. The problem arose while trying to reroute the last few segments. As it happens, the AA rtw desk people were on my side, saying the ticket should be reissued without change of the base price. They were overruled by tariffs. I wound up simply changing dates and was charged nothing.