1. Deactivate the Classic Editor plugin.
2. Open the page.
3. Edit the SiteOrigin Layout Block, click the Layouts button then Import/Export > Download Layout.
4. Reactivate the Classic Editor plugin.
5. Edit the page again, delete all contents, click on the Page Builder tab.
6. Click the Layouts button then Import/Export > Import Layout. Import the file that you downloaded in step 3.