You are correct that it's a contract and as said in terms of service Skype needs to send out notification 30 days in advance.
Subscribers received email notification on the 16th of February and change took place after 16th of March depending on each user recurring payment date.
If you didn't receive the email then please check junk mailbox as it might have ended up there depending on spam filters.
In case you have checked spam folders and your email provider doesn't have spam blockers then please contact Skype customer service:
https://support.skype.com/faq/FA1170Andre
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