I have purchased a RT SCL to PUQ. The return is a base plus fare that allows changes before flight-departure time for a penalty fee, but no changes after flight-departure time. My question is whether "flight-departure time" here refers to the time the outbound leg leaves (so that once I reach my destination, no changes will be allowed at all) or whether it refers to the departure time of the return leg itself, so that I could make changes up until the return time if I pay the penalty fee (plus any fare differences of course).

I am trying to determine if I should pay a change fee now before going on the first leg in order to move up to a flexible fare class, since there is some chance I will need to change the return. I know I should have booked the flexible to start with but unfortunately it's too late for that. Thanks for any help on this!