I went into my Local Verizon store with my current active Galaxy S4 and told them I wanted to activate a new nano sim with adapter in my S4. They said they could not do that because their system will not let them activate a "nano sim" in a phone that takes a "micro sim" and they could not over ride this in their system. I have no idea if that is true or not but they seemed to be trying to be helpful. I went home and took out a scissors and cut down my micro sim to a nano sim and put it in my new N6 it is working fine. I do think however that Verizon activations, if you don't all ready have an authorized compatible sim card, are going to be problematic.
You could have called Verizon had them ship you a SIM card for an htc m8 then activate it on your current line using the nano to micro adapter. Then once it's activated pop it in the nexus 6.
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