I know the rules of a same day flight change do not allow nonstop to connecting or vice versa (or even changing where the stop may be).

I’m flying IAH-LGA and want to switch to an earlier flight which can only be done via a stop in DFW, ORD or CLT.

The EP desk has made it clear that it would have to be repriced if I were to make that change.

My question: is it possible to do this at the airport meaning, if an agent wanted to at IAH he/she could put me on standby connecting through one of those locations or do their systems not even allow that to happen?