Hey sorry it took so long to get back to you!! I wouldn't recommend doing such a big water change while they're still so young! It's much better to do daily small water changes then one huge one. Betta fry stress really really easily, and doing such a huge water change is likely stress and possibly kill some of them!
As for "dry" foods, at about 2 months old you can start feeding frozen brine shrimp and frozen cut up blood worms. As for pellets, they're usually not big enough to eat them until at least 3 months old.
Ok. Sometimes fry will eat till they pop so if you're worried about this just use a turkey baster (new, never used near soap) to regulate BBS. WARNING: Microworms and vinegar eels aren't as nutritous. Have you ever seen a betta with no ventral fins? Thats caused by mal-nutrition which is caused by feeding microworms or vinegar eels for to long. I reccomend you feed them those for the first week then mix it with BBS.