Best to check at the airport for this kind of change. Try to get there early and try to scope out the loads on both substitute flights before you go.

I have had agents do this kind of thing for me (and also offer the reverse--change a connection to a direct flight), but usually they wanted to see both flights have space available so I didn't get stuck in the connecting airport. Routing changes may require reticketing the flight, which is more work for them.