Dear Worker who shows up every day:
Sure you thought you were going to keep your accrued vacation time that you've saved up over the years. Guess what, we are giving you a few days notice that we've changed the rules. Don't like the new system? There is the door--we need to push more people out.
Dear Worker who is gone every few days:
You have nothing to worry about, since you have no vacation accrued anyway.