When should you reencode or mux an old video?

[Guernsey]

Lets say you have an old video file that you need to update and/or remaster, what methods should you use to remaster that that video? Should you mux the file or should reencode it with tools such as FFMPEG and AVIsynth?

[davexnet]

Remuxing the file doesn't change the video in any way, it just repackages it.
If you want to "remaster" (for example, change the size, denoise, fix the color, etc,etc) you have to re-encode

FFmpeg can be used to remux or re-encode, Avisynth implies re-encode because it passes uncompressed
frames to the host program (eg. Virtualdub, StaxRip, Ripbot264, Xmedia Recode, etc,etc)

This is a pretty basic understanding, something you should get clear

[Guernsey]

I see. So you can restore old files using those programs?

[davexnet]

Yes. FFmpeg has some filters built in (eg. denoise), Avisynth has some basic internal functionality to manipulate
video and audio but it's real strength comes from the hundreds of external filters (which have to be downloaded
separately and stored in the plugins folder) written for all the special
situations that arise with videos; quite a few of them are for restoring and fixing video problems

external filters:
http://avisynth.nl/index.php/External_filters
internal functions of Avisynth:
http://avisynth.nl/index.php/Category:Internal_filters