1) You can keep a Verizon unlimited plan even with a contract upgrade. Once you have it you have it as long as you keep a data plan. If they screw up and put you on tiered they'll even roll you back to unlimited if you call customer service.
2: If you can find someone that will transfer an existing line of service to you then you can do it, although Verizon treats it pretty much like a new account from a setup standpoint, ie: credit check, etc.
I have 5 lines on Verizon and have been a VZ customer since the late 80's when they were Bell Atlantic. I've had lots of dealings with them so I know them well. Now if I could only diminish my $350 a month cell bill!
What I'd like to try is adding another line of service to my family share plan and whether they'll let me add unlimited data then to what is essentially a new line, when the master billing line has unlimited. $9.99 + $29.99 for the data plan less my 20% corporate discount would be fine with me for unlimited LTE. Alas though I have 5 lines on my plan which is supposedly VZ's maximum on a personal account (not including data only devices)