Hello @Liam Fermoyle
Thank you for reaching out to us.
In regards to make the synced Accounts cloud only and no longer managed by local AD and changing their source of Authority to Azure AD you need to run the following PowerShell command below.
Connect-MsolService and sign-in using Global Administrator account.
Set-MsolDirSyncEnabled -EnableDirsync $False
After running this cmdlet, you should be able to manage/modify/delete synced accounts via Azure AD. And if you are no longer using Azure AD Connect then you can disabled it or uninstall it.
when you run the above command in order to make effect it takes from 24- 72 hours.
Coming to your second question once you make the users cloud only its not possible for them to be connected to local AD.
In case you have any questions on the same, you can surely let us know and we will be happy to help you further. If this post provides you the answer you were looking for, do accept it as an answer in the interest of community members with similar queries. If this does not answer, please ask further in the comments and we will happy to address your concerns.
Thank you.