Username change implications?

Have an environment that is using Okta for SSO and MFA. We sync from our Active Directory to Okta with no unique additional attributes.
None of our applications have provisioning or deprovisionig setup, it’s all manual as far as user accounts in third party systems go.
My question is, what are the effects of changing the user’s username and email in AD? Since this sync to Okta, will this only affect Okta but not change usernames to other apps? I don’t need usernames changed for SAML assertion. That’s supposed to go unchanged for the time being.
And will this affect the Okta Verify app?