The primary overall effect, good or ill, is to sort of even things out among those of varying years of service. It turns out it's pretty good for the peons who were getting just two weeks a year, because they are now getting more. It's pretty lousy for the old-timers who've maxed out at 5 or 6 weeks because that's going to be tougher for them to keep hold on.

Here's how I would approach it handed this scenario: I'd go to my manager and say
"I plan on taking between X and Y vacation days this year because that's what I normally do, and every few years I'll take a 3 month sabbatical (leave of absence). I'll let you know as much as possible in advance about the time off, as usual."

If I'm stuck with a poor manager who can't handle this, then I look for a new boss or a new job.