I never had any open ticket changed for a date. Just make your reservation, show up at checkin. Allways worked for me.

One advantage issuing a ticket thru a travel agent (and not thru an airline direct) is: the agent can make reservations for you.

When buying aticket i have all the coupons (except the first or the first intercontinental on star) issued open, the travel agent also makes the reservation. I email him when i need changes or additions - and he emails me the new itinary.

Then i just need to show up for the flight, check in. No time on the phone, no time to explain what kind of ticket you have or beeing transfered to the international desk (like when you try to make reservations with AA) etc.