Leaving the December 31 date and grandfathering the May-Dec 2005 preserves 12yo year for those 8mo of players, but essentially takes it away from the May-Dec 2006 players since they will be at the tail end of a 20mo long age group. Building another opportunity for those kids is the right thing to do - though I'm at a loss in terms of how to do it properly.
It's true, the 2006 players would be getting the short end of the stick.
As a league you can add a second 9/10 tournament team in 2015 and 2016. (With DA approval.) Give 2006 players preference for the second 9/10 team in 2016. They may not fare well, but they might have some fun and improve for future years. The same thing can be done in 2017 when the 2006 players are LA11, add a second 10/11 team and give 2006 players preference on that 2nd team.
2018 is up in the air right now. LL will obviously have a plan by then. I hope an additional Majors tournament team is part of the plan. Maybe that would make for a HUGE long tournament? Maybe there is another solution?
Grandfather the 2006 players, born May - December, into the 2019 Majors division and the 11/12 tournament? A smaller percentage would return in 2019, reducing the impact on 2007 players. Maybe create a rule that if they are on a 11/12 tournament roster in 2018 they are not eligible to return in 2019? Of course this would be a pretty cumbersome solution.
In the meantime districts can also create a special games tournament for these 2006 players.