API-driven provisioning field mapping changes resynchronize all users and groups

We have configured API-driven provisioning for on-premises Active Directory, along with Azure AD Connect, to synchronize on-premises AD users with Azure Entra ID.

As part of the provisioning setup, we have used a separate Organizational Unit (OU) in on-premises AD (designated as the default OU for new users) while configuring API-driven provisioning.

We are attempting to make some changes to the API field mapping, specifically the ‘UserPrincipalName’ regular expression (custom domain) and the ‘manager’ field, and saving the configuration. Upon attempting to save, a prompt appears (as highlighted below screenshot), indicating that this action will resynchronize all users and groups.

Could you please clarify:

1. Will this resynchronization update any existing users outside the default provisioning Organizational Unit (OU)?
2. Specifically, what does the resynchronization operation update? For instance, will it modify the 'UserPrincipalName' and 'manager' attributes for all users including old users outside of provisioning Organizational Unit (OU)?

Screen Shot - While Saving Mapping.