The concept of co-terminals relates to the rules governing allowable routings on a nonrefundable ticket (roundtrip, open jaw. circle trip). For example, it allows you to fly into one airport (e.g., LAX) and fly out of a coterminal (e.g., BUR) without it counting as a stopover when you originally purchase your ticket. You could fly PHL-LAX/BUR-EWR and that would be a legal open jaw. In this case, you would pay half the roundtrip fare PHL/LAX plus half the roundtrip fare BUR/EWR.

However, since the fares themselves are often different from different coterminals, the rules governing a change from one coterminal to another after you purchase your ticket are different from the rules changing just the time of return from the same airport. For example, if you originally bought a roundtrip ticket from LGA to LAX back to LGA, then you wanted to change the return to go from BUR to LGA, you would be allowed to make that change because they are co-terminals. However, you could not just pay the $100 change fee.

You would have to find the lowest applicable fare from BUR to LGA at the time you make the change and pay the difference between that fare and what you originally paid for LAX to LGA segment(in addition to the change fee).

That is why it matters when you make the change. For example, your cost in making the change would be quite different depending on whether that change was made 14 days in advance, 7 days in advance, or 1 day in advance, since different fares would apply on the BUR-LGA segments.