Impact - Enabling SAML/SSO after Users are active already

Normally we rollout DUO tenants with SAML/SSO before enabling Users.
However, we found 2x Tenants which have not been enabled for SAML or SSO, and these clients are using DUO already.

Question: What would the impact be if now proceed to enable SAML/SSO?
Referring to enabling below with Azure AD:

The impact would be that the users who already exist in Duo would also be able to authenticate through SSO for any SSO service provider apps you then configure after enabling Duo SSO.

If there is a concern that the Azure usernames don’t match the usernames of the existing users, the Azure usernames can be added as username aliases to existing users in Duo via Azure directory sync, CSV import, Admin API update, or manual entry. See this guide.