Hi Erhan,
When appropriate you should be able to convert to Pay-As-You-Go which will allow you to purchase Savings Plan. Please see "Term & Conversion to Paid Account" section of below article:
Microsoft Azure Sponsorship Offer
https://azure.microsoft.com/en-us/pricing/offers/ms-azr-0036p
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP