It'll have little or no effect on the kids with August or September birthdays. The negative effect will be for the kids with the March, April birthdays just like it always was for the June, July birthdays. It will end up losing a lot of players in the 14yo age group because now a 4th of the kids who are considered 14yo are that much more likely to be playing HS ball. You'll also see a LLWS where the kids are predominantly 13yo instead of just having a few 13yo on each team. The LLWS will be even more of a mismatch as the best pitchers will be throwing 2 or 3 mph harder at 46' and will simply throw the ball by the hitters even more so than they do today.
All that has happened is that you've switched who it is good for and who it is bad for. Nothing has been improved. You've traded not having 11yo for having kids who are 13yo for their entire season. The difference between a young 11yo and an old 12yo is less than the difference between the youngest 12yo and the oldest 13yo you'll see with the new age group because you are more likely to see differences in maturity level as the kids move into puberty. You've increased the disparities within the age groups.
The age change itself is a small negative. When you add in the confusion and trouble implementing the change will cause for the first year or two it is very definitely a mistake.