So I tried just yesterday at Dusseldorf airport.
Booked a PLUS ticket via Expedia, and the counter check-in agent refused to move my flight, claiming the ticket does not have the suitable provision to stipulate free same day flight change. Got referred to ticketing desk person and the answer was still no.

Looks like the fare rule on the GDS tickets don't say explicitly same-day flight change is free, and contracted out port agent would not let me through.
Ended up having to call BA call centre to make the change, and the call centre had to reissue the ticket altogether.