Now we are using Azure AD Premium P1 and we want to step back to Azure AD Basic without affect Office365 as it was before Azure AD Premium P1.
Is it possible?
It's possible, you will simply loose access to the corresponding functionalities that require P1. All your objects and configuration will remain in place.
If you we're using Azure P1 trial licences or bought licences - just wait they expired. Be sure to have no automatic renew options. Also, when you're saying "I have 100$ credit every month" are you speaking about Free tier Azure or just you company that allows 100$ / user / month ? - In the first option, Azure free tier has nothing to see with Azure AD Premium licences. It will be counted separately.
Do not forget, it's also possible to have MIXED scenarios. For example, a lot of customer will take Azure AD Premium licences for VIP or risky people (Admins, VIP, etc.) to get all the security options (MFA, PIM, etc.) and will keep the base or a lower option for all the "standard" people.
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