I'm guessing that CBS wants the show in HD, and I'm guessing that they don't particularly want to lay off all the extra CBS-Digital staff who have working on Next Gen for the past three years... so from a logicistical perspective, I would say if DS9 HD is going to happen, it's going to happen sooner rather than later, striking while the iron is hot. They might even be scouring the vaults for the film reels now.
Totally optimistic guess on my part, but hey, what's the alternative? They finish TNG and everyone stops working and goes home?