How to go back to the old YouTube playlist editor?

Question

The new YouTube editor doesn't have all the functions of the old version, the function to add all to another playlist is no longer available.

Answer 1

I found that it is possible to add parameters in the Url to disable the new design.

&advanced_settings=1&disable_polymer=1

For example to edit your Watch later playlist

https://www.youtube.com/playlist?list=WL&advanced_settings=1&disable_polymer=1

Then you get the small button more button at the top right with "Add all to..."