I don't think you have to redeck the roof. My house is decked the same way and has had drywall on (parts of) the underside for years.
As long as the roof is watertight, you should have no problem. However, if a leak does occur, it may damage the ceiling faster than if the roof had plywood sheathing. But I wouldn't worry about that.
I'm assuming you're familiar with the styrofoam chutes that need to be stapled to the underside of the roof before you insulate. They let air flow between the insulation and the roof. They may also provide *some* protection from a minor leak.