Good luck -- I vaguely remember seeing things where this situation will backfire or they closed the loophole or something. Try playing with it on your account, take screenshots the whole way, and don't hit go if it looks like they are taking away your unlimited.
And assuming the system seems to let you do it, activate the phone first on the secondary line (maybe leave it on overnight so you are sure it shows in the system) and then put the old dumbphone back on (remove the data).
You may then need to go to a store to get a new SIM for the primary line (or, throw your existing SIM in if you have one already). I had one store rep refuse to do this for me, and went to another store and had absolutely no problem. If you already have a SIM for your line this step should obviously be super easy.
I did the regular "use the second lines upgrade on that line and then put the new device (after above shenanigans) on the unlimited line" this summer and it worked fine.
edit: and if you can, get one of the new secure element SIM's while you are at it just in case you ever need one.