What is the process for converting an old movie to high def (1080p)?

Question

I just dont understand how a movie that was made before hd was around is converted to hd.

Answer 1

Movies are recorded in film, which is analog and of much higher resolution than even 1080p.

Studios use special film scanners (called telecines) that take the film picture - and record it in the desired resolution.

Answer 2

Long before even dvds were around, movies were shot at a much higher resolution than personnal equipment was able to display.  

So for the most part they're going back to master copies and putting more of the original resolution on the newer HD formats.