Usernames for Existing WordPress Users

I am setting up a community for an existing membership site that has many people with WordPress user accounts already. It looks like their name shows up as their email address when they are added to a space (so if you look at the display name, you'll see [First Last] but if you try to tag them or look at their profile, you'll see [nameexamplecom] with a simplified version of their email address.

Is there a way to change that in advance of adding people into the new community? I don't want to expose their email addresses by default, and for the last 14 years we've been setting up their accounts using their email address as their WordPress user name.

Or will I need to basically delete everybody's email accounts and then re-create them as I go along and import their billing data into FluentCRM? (We're currently on ThriveCart but are going to migrate into FluentCart for new subscriptions.)