Raymond Rohauer made a practice of altering the original films in order to secure a new copyright. He would change the titles so that they were in a new lettering style and with minor changes of wording. Such changes were called grounds for claiming that new creativity had been expended and that the new prints were protected by separate copyright. Adding a soundtrack to a silent film has long been grounds for claiming a new copyright on a public-domain movie, although it seems understood that if the picture is unchanged, someone else can strip off the soundtrack and legally duplicate the movie.
Restoration is a trickier area.
It may seem an outrage that long hours and exorbitant expenditures would not be rewarded by legally-enforceable exclusivity. I can tell you that the Supreme Court ruled earlier in this decade that telephone directories were not eligible for copyright because no creativity was involved in what the court saw as a mere compilation of factual information that was readily-accessible to the telephone companies. It was shortly after that ruling that multiple brands of CD-ROM editions of the U.S.'s phone directories began to appear. Right now, there are many such CD-ROMs, and can be bought for very low prices at local retailers. At the time of that ruling, I was working for a distributor of computer databases.