What does "new construction only" mean for Recessed Light Housing?
Unless there are other restrictions such as clearance to framing members or insulation or open space above them you can use them.  "New construction" is when the framing is open and not covered by drywall*; the fixture is typically attached to a framing member.  As opposed to "old work" which is where it's closed on both sides and you're going through a small hole in the drywall; the fixture is often attached by clamps to the drywall itself.

* With some "new construction" fixtures you need to have the wall open on the side you're working; with others you can get away with having just the other side open because the fixture will give you enough room to work in.