A) Yes – you can switch from on-demand to Reservations on the fly.  

Azure Portal >> Navigate to All Services >> Reservations >> Click Add >> Define a scope and create.

You don’t assign that reservation to a specific VM, rather, the discount is applied at the subscription level  automatically

Q2) Can I upgrade the size of my reserved instance?

A2) Yes, you can increase / decrease the size of your reservation (entails a restart).

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