Hard match between onprem and cloud users

Hi,

I have installed new Entra Connect (with ABA) for the customer. PTA and SSO will also be used.

Right now users basically have two different accounts, that share the same UPN address, the on-prim and the cloud account.

My questions are :

1 - Users already have licenses and are actively using services such as MS Teams, Outlook, OneDrive.

Will there be any negative impact on users during or after the hard match process?

Or is there a risk of data loss?

Does a prior announcement need to be made?

Some users may have different UPN and email addresses. Is there any negative impact after a hard match?

How do I get this Azure AD Connect to correctly merge my accounts?

Thanks,