Cannot change my VM from A4 to D4 size.

I have several VMs in Azure in the same subnet. I cannot change the VM size in one of them from A4 to D4 size. When I change the Virtual Machine Tier from Basic to Standard I do not see any D series configuration in the Virtual Machine Size drop-down; only A size. For my other servers when I select standard tier I see both the A and D series sizes.

Can any recommend what I can do to make the switch: I need to increase both the size of the memory as well as get the faster CPUs in D series VMs.

Please see the image: (will upload the image once my email is verified).

Thanks for any help.