Force existing users to accept terms and conditions

I have not used the 'Terms and conditions' field on my website until now, but with the new GDPR regulations I would like to start using this.
The setup itself is clear to me, but my problem is that I already have a lot of users that were not requested to accept any terms and conditions when they registered.
How can I now force my existing users to accept these new terms and conditions next time they logon?
Or in other words, how can I force my users to update their profile at next logon?

Best I can suggest to partially automate this is CB Auto Actions redirecting to profile edit if their terms and conditions field hasn't been accepted yet. Once we integrate with Joomla 3.9 component that's meant to handle GDPR that'll probably be able to take care of this as well, but I've no idea when that'll happen as J3.9 isn't even released yet. Your alternative I guess is to give a deadline for them to review and accept the terms and anyone who hasn't done so should be completely purged from your site.