We generally recommend users to use their email address to log in to the new forum for the first time because the import changes a (generally very small percentage) of usernames because the Discourse username rules are more strict.
So if someone has forgotten the email address they’ve registered with, they will be able to login with their username/password in 99% of the cases if you are using the migrate paasword plugin. And in the rare case they forgot their email address and their username belongs to one of the few that were changed, you’ll only need to tell them the new username that corresponds with the old username. No convincing needed.