I don't think you'll have any problems transitioning the tank. I'm glad to hear you've decided on upgrading. Your betta should show more signs of activity, since there will now be a current, and filtration which will keep the water clean and to his liking. If they were doing fine before, they will love it now. Although with a current now in the tank, you may not see a bubble nest again, depending on the flow rate. In a 2.5 gal though, I doubt there will be dead spots on the surface for him to make any.
Keep us updated, good luck.