Hi,
That option or feature can only be disabled or set to enabled/disabled by the Global Admin at the organization level, not sure what you mean change the Combined Registration if it is enabled it is for all the users.
The benefit of CR is with combined registration, users can register once and get the benefits of both Azure AD Multi-Factor Authentication and SSPR.
After you enable combined registration, users who register or confirm their phone number or mobile app through the new experience can use them for Azure AD Multi-Factor Authentication and SSPR, if those methods are enabled in the Azure AD Multi-Factor Authentication and SSPR policies.
If you then disable this experience, users who go to the previous SSPR registration page at https://aka.ms/ssprsetup are required to perform multi-factor authentication before they can access the page.
Hope this helps.
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