Thanks for your post and sorry you're experiencing issues with SQL Monitor.
One hypothesis I have is that the account the "SQL Monitor 4 Base Monitor" service is using has changed. (There shouldn't be anything different about the data collection methods between the two versions.)
You can also check on the monitored servers page (in configuration) to see what the failures are. If you hover over the entity that isn't connected, there should be a "show log" link on the right. That should tell you which data collection methods are failing and what the error messages are.
If you'd like to troubleshoot further, we have a ticket open for you (#23640), so feel free to respond to the email you should've gotten from our support system or email firstname.lastname@example.org