There’s no built in feature to do exactly what you’re asking for here, so let’s first explore “creative solutions” territory.
First, a question about this:
Are you hoping to allow them to continue participating in the community using the same account? Or will it be required that they create a new account if they wish to do so?
If you’ll be “owning” their existing account and require them to create a new one, then you control the email associated with the account. In that case, one approach here is to use an email based strategy – you could change the email address associated with their account to one you control and have email notifications from Discourse forwarded to a group that is responsible for your community.
I can imagine other similar creative solutions, but would need to tinker or explore more to validate whether they are doable today – for example, using Discourse Automation to pipe things into a chat channel your team can see, or something like that.