- Go to Watch Later list with Advanced settings
- 3 Dots Menu in top right > "Add all to" > Choose your destination playlist.
- Ensure it's done by visiting the detination playlist and then proceed to Delete your current 'Watch Later' to add new set of videos to it.
Long Answer: (with additional old and hopefully still working method)
I originally meant to follow this guide from 2018 by nouyang.
But at the point where it says to "Finally, go to your “watch later” playlist" it redirects to this regular Watch Later link, https://www.youtube.com/playlist?list=WL
This is also the URL available for "Watch Later" in your YouTube's left panel. (The panel you get by going to harburger menu in the top left).
Unfortunately, if one is to access their Watch Later via this URL, they won't find the '3 dot' to choose the "Add all to" option, which is important in moving videos from Watch Later to a playlist.
So, i was planning to follow this lengthy but resourceful guide from 2014 involving YouTube API, which also happens to be suggested by @Kangaroo here.
Thankfully, i was taken aback by the lengthy process and with some thinking & trial and error, i discovered this Watch Later URL with advanced settings which includes the beloved "Add all to" option: