You can't switch a New editor lesson over to the Legacy editor in place, but you can make a Legacy editor copy of it in one click. Your New editor lesson stays exactly as it is.
Making a Legacy editor copy
1. Open the lesson, click the three dots in the top right and choose Convert to V1.
2. The dialog confirms that this makes a new copy and leaves the original alone. Click Convert.
The copy opens straight away. It is built from your latest edits, including any you haven't published yet, and it is saved to My Library as a draft with a label on the end of its name, so you can tell the two apart. If the original has a tutor, the copy gets its own copy of that tutor. Publish the copy before you assign it.
Nothing happens to the original. It keeps its pages, its classroom assignments and its students' work, and you can carry on using it.
What changes in the copy
The two editors do not have the same blocks. The Legacy editor has no page structure, so the pages are joined into one lesson and page context goes, and anything without a legacy equivalent is converted to the nearest thing. Check the copy before you assign it rather than assuming it came through intact.
Most people do not need this. The New editor is where new features are being built.