[chiph]

A good example of this was the recreation of Star Trek: The Next Generation in Blu-Ray resolution. The main scenes were still available on film and could be rescanned. But all the special effects (phaser beams, transporter sparkles, etc) had been rendered using late 80's technology. And had to be recreated from scratch.

Some of the model shots also had to be entirely redone via 3D effects, such as the Crystalline Entity in S05E04. Paramount spent millions on this project, which were never recouped as the discs were released just as streaming took off.

(As a TNG fan, it was totally worth it.)

https://www.youtube.com/watch?v=2k7R_nYueBg