If my headliner has no rips/tears/stains, but is hanging, can I re-glue the original headliner down?

I'm nervous that I may not be able to find a close enough match at the fabric store.

 

What color is your A-pillar? I had a beige A-pillar and bought black material at the fabric store. The contrast in color actually made the car look really sharp. 3M spray adhesive worked awesome for me. It comes in a can and sprays out orange.

To answer your question though, after my headliner sagged for a month, it took a permanent curved shape. If I tried to re-glue it, there would have been wrinkles everywhere.

 

Gluing alone seldom fixes the problem as the foam backing has generally deteriorated almost to a dust which is the cause of the adhesion failure initially.
In the states you seem to be able to get headlinings professionally done for significantly less than one can here in NZ.
I recently had several quotes all around $650.00 to redo mine, hence my hesitation at the moment. Sunroof's make it more labour intensive too.
Large range of fabrics around so getting a close match will never be a problem for you.

 

My 850 wagon has headliner issues too, the leading edge above the drivers seat was hanging low. The foam backing is pretty dust like, showering you in beige powder if you touch it. My father in law had some upholstery spray glue, my wife sprayed it one and reattached the headliner (I'm a bit hamfisted at this sort of thing) . We did it this past Xmas, and so far so good.