My experience is that Topaz Video AI gives you the improved video and copies the audio track only to the output.
So I just want to mention here one solution. All my video files are either .mkv or .mp4. The following works for either.
There’s a easy way to replace the video track on the original input file with the video track produced by Topaz thus retaining all the original subs and other tracks in the original file using MKVToolNix mkvmerge.
You can get this free (MKVToolNix download latest version)
You can do this using the GUI but the following CLI command example is one I used to replace the original SD movie with the Topaz upscaled HD Abyss video while retaining all the other original tracks like subtitle and chapters:
mkvmerge.exe -o “C:\merged_videos\The Abyss Extended (1989).mkv” -D “D:\The Abyss (1989)\The Abyss Extended (1989) 480p.mkv” -A -S -T -M -B --no-chapters --default-track-flag 0:1 “D:\The Abyss (1989)\The Abyss Extended (1989) 480p_amq13.mkv”
What this does is merge only the video track from the Topaz enhanced file with all the other tracks except the video track of the original file producing a new output file. Neither the original or Topaz file are modified.
I have a Python script to walk a directory in order to do a TV season of files for example after I process them all with Topaz.
Hope this info is useful.
Note: There were a couple errors in the command line flags in my original post. Those have been corrected with this last edit as some wording to better describe the process.
