To Gamepress : regarding Community Day

The wording of the original tweet suggests that any Dragonairs that are evolved in the 3 hour window will get Draco Meteor

''Great news, Trainers! If you're lucky enough to evolve your Dragonair into Dragonite during this month's #PokemonGOCommunityDay, your Dragonite will know a special move—Draco Meteor!''

It says ''your Dragonair'', so that makes me think it applies to the Dragonair you've been walking for 6 months too

Most likely the dratinis/dragonairs to be evolved don't need to be caught during the 3-hour event window to get Draco Meteor. We will have confirmation as soon as the event starts in Australia/New Zealand

Niantic's language around the announcement has been unfortunately unclear. We cannot say for certain whether or not Draco Meteor will only be obtained only on Dratini obtained during Community Day. It's likely that we won't know until the day of the event.