Upgrade on another line question

[ManBearPig]

I actually have my upgrade open, but I was curious about this, so here's a theoretical question:

I'm on AT&T. Let's say I have a smart phone, my dad has a dumb phone w/ no data plan, and he has an upgrade but I don't. If I use his line to upgrade to a smart phone for me, would we both have to have a data plan even though he'd still have a dumb phone?

Thanks yallz!