Can I force existing members to re-register when they next log in?

[chapoi (Charlie)]

@jahan_gagan

users who attempt to save their profile or update settings will be prompted to fill required fields. However, this does not interrupt normal browsing and only triggers when they enter their user preferences

That’s not correct. The modal clearly warns you:

And the when logging it it shows:

And the site is usuable until I do.

So yes, the “Field Requirement” setting is the right approach, but you don’t need the other setup to force it.