Last year, to be eligible to play in HOCO 7th grade (2029 class year), your birthdate could be no earlier than June 1, 2010. That was the 15 month window. Now, to be eligible to play HOCO 8th grade (2029 class years, your birthdate could be no earlier than June 1, 2009. In other words, they add an entire year to the 2029s.
The rumor is that Cabell wanted the option of pushing his 2028 son down into the 29 year and pushed HOCO on this because his son isn't good enough at the 28 level. Whether this is true or not I have no idea but it sounds like something he would do.