Hi Andrey Khmyrov,
Thank you for reaching out to Microsoft Q&A forum.
Yes, it is possible, you can start with pay-as-you-go and switch to reserved instances later. So, you can use pay-as-you-go for the first couple of months while you figure out exactly what you need. Once you’ve got your VM sizes sorted, you can go ahead and purchase reserved instances for those sizes to save some money.
Just remember if you choose to upgrade to larger VM sizes when you switch to reserved instances, your overall costs will go up.
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