Open the page, click Use New Builder, then Import > Import the design from the old builder. Every module currently on the page is added to the canvas as a placeholder block that keeps the module's saved settings. Review each block in the Inspector, click Save Page, check it with Preview, and finally click Publish Page. The dialog warns that some legacy modules may not have all their options available in the new builder; if a block does not look right you can keep the page on the classic modules by leaving it unpublished. If the page has no modules you will see "No DIY modules found to import".