I tried this last month with a connecting flight in Calgary (flying in from LHR)

When I checked in at LHR and asked to be moved to an earlier flight in Calgary the check-in agent wanted to charge me $200. I declined the change, there could have been delays or who knows what and I've wasted the money, the agent advised me to check again in Calgary.

When I arrived in Calgary at the transfer desk they checked my details and I asked if there was an earlier flight I could take, the check-in agent worked his magic and put me on an earlier flight, with no charge. The earlier flight left 20 minutes later so my time on the ground in Calgary was about 40 minutes in total. I made it to the gate just as boarding commenced and was home 2 hours earlier than expected.

So... in summary, it does seem to happen.