We have always considered our "season" August to July since as long as I have been involved. In fall, every second year team moves up. It hasn't ever been a problem that I have encountered.
Since travel teams are generally not ever made up completely of girls from the same school, what grade kids are in doesn't seem like it would be a big deal. When teams get to 16u, they usually put their grad year on their jersey sleeve anyway, right?
Really it just changes who is the "oldest" and who has the "bad birthday for softball". There will always be the girls with unfortunate birthdays, right now it is December, November. I have a daughter born in August, and her best friend is born in September. They would now become the players no longer on the same team, but in the same grade. It will just alter which girls are caught by the age cut off, I suppose.
The majority of us travel teams (in my area) are not Showcase teams. We are simply helping girls use softball to build their confidence as young women, learn valuable lessons about being part of a team, learn how it feels to win, and to lose, and most importantly how to act when that happens. So keeping grad years together to help college showcases doesn't seem like it should need to affect the whole travel ball system.