If Dudley was a wizard that would have definitely changed Harry's upbringing. There would have been other forces at work early on helping Harry prepare for what would eventually happen and something like that could impact everything of the books even to the extent of James and Lily surviving. Had Dudley been a wizard then Lily and Petunia would have been closer and she could possibly have been the secret keeper. If James and Lily had survived the events and outcome of the first wizarding war would have been far different.

Steph//Puff