I have managed to do this once, but it must have been four or five years ago. I was flying from TLL to HEL. They had their own check-in and ticket office staff at TLL then and the flights were also AY-operated.

My flight would have left at 15.20, however I got to the airport in time for the 14.20 flight and asked nicely at the check-in. They directed me to the ticket office, where the agent was happy to change my booking with no hassle at all.

The TLL flights are usually only half-full anyway, so it shouldn't be a revenue problem, either.

By the way, this is the only time I have asked, so my success rate is 100%