Your Verizon VM is hosted by Verizon. When you switch to GV, you'll be using a box on their servers. THey'll be separate. But, they won't merge. Though, there may be a way to forward a message from your verizon VM to your google number. Once you set up a GV number, that's what I'd try... forward your current verizon voice mails to your GV number. Not sure if it'll work, but I can't see why it wouldn't.
This might work. Ok maybe. Go back to verizon voice mail. Forward the messages you want to save to a friends phone. Then re-set up google voice and have your friend forward them back to you. Just a wild guess. Let me know if it works. There's prob an easier way, I just don't know it.
To disabe Google Voicemail dial "*73" and call it,it will hang up by itself,to re-enable Google Voicemail dial "*71 + Google Voice access number(Google #)" so it would be "*71_ _ _-_ _ _-_ _ _ _" and it will be back to google voicemail.