We want to update our ArcGIS Online presence to use our company's Active Directory logins for accounts. The problem is, we already have dozens of accounts already made for specific people using the built-in account system. Will enabling the AD logins break the use of the built-in ArcGIS Online accounts?
No, enabling the enterprise logins will not break existing accounts. As users sign in/accept invitations using enterprise logins the organization will populate with new user accounts.
Keep in mind that all accounts are unique across ArcGIS Online, so you cannot duplicate an org account with one created through the enterprise id login process.
If you want to keep using both types of accounts in the org, you can. Or if you like you can change the ownership of items in the org from a built-in account to the new one using the tools in ArcGIS Online. Or you could use a tool like this
If you have many items to move and group memberships to update.
Once items are moved you may disable/delete the first account.
David is correct after enabling ADFS you will be left with both accounts, the original account and a new account under AD. Here is a blog post that we wrote explaining how you can " Migrate the original User's Content to the newly created account after enabling ADFS " as well as how you can bulk delete these users once the content has been migrated.
Hope this helps.