Account level IAM vs IAM Identity Center

We have multiple AWS Accounts that all have their own individual IAM Users/Groups and permissions. These are all from acquisitions. We've created an AWS Organization and enabled all features.

My question is, when we add an external IDP for SSO, will the Users/Groups at the Account level IAM remain intact? Based on the documentation I believe they will, and at this point we can start migrating the Users/Groups out of the Account level IAM and into the Org level SSO?

Some of these accounts are critical and I just want to be really clear before I potentially make a huge mistake.