I've reviewed discussions on username vs email for login form but couldn't find any reference to a similar case.

The website I'm working on has large number of existing clients who can only use 'assigned' username to log in to the site.

However, brand new users can use their email address as their username.

We thought of a few label options:

'Username or email' We worry if existing clients will attempt to log in with their email which they're not allowed to (we have a long term plan transitioning them to opt to use email address instead of assigned username - but not anytime soon).

'Username' Technically this refers email address as in 'username', plus legacy clients' username, however, it is not clear for brand new users who don't have assigned username.

Any thoughts on getting around this gracefully?

Thanks in advance.