Forcing a user to consent to terms that were created AFTER their registration in pure Joomla would require that user to edit their profile. I've seen users accounts that were NEVER updates since their creation, so waiting for a profile edit to receive consent is a waste of time. This is one of the reasons the System - Required Fields plugin was created.
Using this plugin, you can select fields which will be tested upon login and if they aren't complete - the user is directed to their profile edit screen where they cannot leave until the required fields are populated.
For GDPR compliance, we're using this plugin to force users to edit their profiles when a new Fields - Terms of Service field has been published and they have not yet consented to it.
This specific plugin is really powerful, and has many uses beyond GDPR compliance. I highly recommend exploring the capabilities of this extension for other uses on your site.