Not exactly. Waiting until after departure generally allows them to use historical fares and original issue date to measure any advance purchase requirements for fares. But if there is no longer any inventory left on any flight options in your original booking class, they won't be to make change without pricing you up to a higher fare class. So if you have a fare with a 21-day advance purchase requirement and change return prior to outbound departure, they will use the change date to measure advance purchase and you will no longer qualify for a 21-day advance fare if it is less than 21 days out (even if there is still inventory in original booking class). After departure, they can use previous issue date to measure advance purchase and you can still qualify for a 21-day advance fare even if it is less than 21 days out to return. But there still has to be inventory in your original booking class in order to preserve original fare price.

There can also be fare changes if you have a seasonal fare and the new return is outside of the seasonal requirement (as shown above). Or if you have a fare specific to a particular days of week (Mon-Thu is a common requirement for cheaper return fares on transatlantic travel) and change to a day of week outside of that.