Yes that will work just fine as well. Also keep in mind, if your unlimited line is upgrade eligible, you can also transfer the upgrade to line 2 or line 3 to get a device at 2yr pricing. As long as the phone is activated on line 2 or 3 first, you will not lose unlimited data.
Hmm...I've been reading on the past couple pages back that if you use the upgrade on the line that has the unlimited data. You'll lose the UDP on that line even if the upgrade is going towards another line. Seems risky unless someone can confirm that this is feasible???