Will changing the Default Library and List experience affect existing sites?

[Stephen Reilly]

My company has acquired another company that was using SharePoint Online prior to the acquisition. Some of their libraries and lists are in the classic experience and they do not want to change them over to the modern experience.

If I change the default experience in the SharePoint Admin settings will it effect sites that already exist and force the classic lists and libraries into the modern experience, or will this only effect sites that are created after the change is made?

The documentation on this setting is not clear on this. I understand that at this moment there is still the option to view the site in the classic experience, but I believe that changes every list and library, that the users that selected this option, view. I also know that we can change individual lists and libraries to have different experiences. I'm mostly curious about if the change will effect existing sites.

Thank you

[#2 · Accepted solution]

It will only change the lists/ libraries that have not had that setting touched. So by default it's set to the "Tenant Default" or whatever it says, So unless someone went in all those sits that dont' want the new setting and changed it manually to classic mode, changing the default will change everyone's setting.

[#3]

So if there are (lists and libraries) that do not want to be moved over. I can go in and set them to the classic experience by hand. And in addition, I can use the PowerShell commands to keep entire Sites and Site Collections on classic. 

Then when I change the Default in the SharePoint Admin Settings those sites will stay classic because they are no longer 'Tenant Default'? 

Am I understanding that correctly?

[#4]

Yes, the default will only affect things set to use the default setting and new items created going forward.

[#5]

Correct!

[#6]

Thank you.