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.
Downloading System - Required Fields
To download the extension, you must be a subscriber to either System - Required Fields or to the GDPR Bundle. By popular demand, I created the bundle to make it easier to purchase all of the GDPR extensions at once (turning 3 trips to Paypal/Stripe into 1). To reward users for buying the bundle, I gave it a 20% discount.
Once purchased, the GDPR Bundle page and the System - Required Fields page will present download links in a section labeled "You are a subscriber".
In /administrator, go to Extensions > Manage > Install. Browse for the plugin you downloaded and press the install button. You should soon see a success message.
Browse to Extensions > Plugins and search for "Required". One of the results should be "System - Required Fields". Click the red X next to the plugin to enable it. When you're finished, it should have a green check next to it as seen in this screenshot:
The only absolutely necessary setting in the plugin is selecting the "Edit Menu Item". This refers to the menu item that leads a user to their user profile edit screen. If you don't have one of these links set up, do it now.
The rest of the settings change how it behaves for different users on the site. The most likely configurations you'll want to make is to Un-Require the TOS fields for your administrators. By Un-Requiring these fields, your administrators will be able to edit a profile without being force to agree to terms.
This is where the "Required" setting of the TOS field comes into play. By setting a field as "Required", it becomes available within the Un-Require fields setting of the System - Required Fields plugin. The best practice is to select every required field in "Un-Require" so admins may freely edit profiles without being forced to fill in details they may not know.