Migration tools to transition your classic sites will be released in 2018. Both classic and new Sites will continue to be supported during this time. Timelines and instructions regarding the gradual depreciation of classic Sites will be released in 2018. The specific depreciation date will be announced at least one year in advance.
Read More *** Note that the help file is updated indicating that site conversion is or will soon be available, even though many features are still unavailable in New Site
After Migrating you may have to add the custom URL to the migrated site.
Migration will fail for some large sites. You can try copying some site content to a new site and see if it will migrate, or maybe just wait until the migration process is improved.
Please note that if you try to migrate a site too often you can be blocked for a while from further attempts.
For a test I created a new Classic site that contains a text box with some text and a table. I then migrated it to a site in the New Format.
NOTE: The spreadsheet is not automatically shared. You must access the spreadsheet and set its sharing to public or anyone with the link in order for it to be viewed by site visitors.
Attached or File Cabinet files will be moved into a Google Drive folder and inserted into your site.
NOTE: The folder is not automatically shared. You must access the folder and set its sharing to public or anyone with the link in order for it to be viewed by site visitors.
You will get some warnings about this when you migrate a site but they are collapsed and easily missed.
Examples:
List pages detected. Your site contains list pages. They will be converted into Google Sheets and will be embedded in the new draft. The Sheets can be found in your Google Drive.
File cabinets detected. Your site contains file cabinets. Files stored in file cabinets will be added to a new Google Drive folder. The new folder will be named after the converted site and will be embedded in the new draft. Links in the file cabinets will be placed on the page above the embedded folder.
Page attachments detected. Your site contains page attachments. Files attached to a page will be added to a Google Drive folder named after the converted site. In the new site, attachments will be embedded in a Google Drive folder at the bottom of the page.