Q. I switched my Azure AD instance from federated to standard authentication but now I cannot login.
A. I recently had this exact problem. I switched from using federation for authentication to standard (authentication in Azure AD) however when trying to login it failed and still redirected my to my ADFS server which then displayed an error. The solution is to wait. It can take up to an hour for the change from federated to standard to propagate through all the Azure AD infrastructure. For this reason if you are going to switch to standard you should do it in a quiet time when there will not be authentications. Also make sure you did not skip the user conversions as this would mean the users would still be configured for federation. Rerun the conversion without setting the skip user conversion to true.