A lesson built in the New editor can be converted back to the Legacy editor if you need it.
Switching back
Open the lesson, click the three dots in the top right and choose Convert to V1.
What you lose
The two editors do not have the same blocks. The Legacy editor has no page structure, so page context goes, and anything without a legacy equivalent is converted to the nearest thing. Check the lesson afterwards rather than assuming it came through intact.
Most people do not need this. The New editor is where new features are being built.

