The answers to your question you can find in the FAQ of Azure Free Accounts:
https://azure.microsoft.com/en-us/free/free-account-faq/
What happens once I use my $200 free credit or I’m at the end of 30 days?
We’ll notify you so you can decide if you want to upgrade to pay-as-you-go pricing and remove the spending limit. If you do, you’ll have access to all the free products. If you don’t, your account and products will be disabled, and you'll need to upgrade to resume usage.
After 30 days you have to upgrade from a free Azure Account to Pay-As-You-Go. That is the rule made by Microsoft.
But you are not charged for free Azure services/products listed on the linked FAQ webpage.
The other option you have is to cancel the Azure free account and create a new free Account for the next period of 30 days.
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Regards
Andreas Baumgarten