Keep my unlimited data

Ok, here's the question

I have 4 lines of service on my plan. All are smartphones. 3 of them have unlimited data and the 4th has 2gb. My phone is available for an upgrade in 3 days and it has unlimited data. Can I transfer my upgrade to my son's line, which has the 2gb, then order the new phone and activate it on my line without changing anything or losing my unlimited data?

That should work as it would be like buying a phone from a 3rd party and activating it on your line. It shouldn't jeopardize your unlimited plan.