upgrade first line on family plan: $10 fee on second line?

Howdy, knowledgeable forum-goers!

I've got two smartphones on a family plan, but I'm not currently paying the $10 monthly fee on either. If I look at my current plan online, it says Premium Data Fee: $0, so I'm grandfathered in on both lines.

The main line is eligible for the early full upgrade (premiere gold). If I upgrade the phone, presumably I need to re-up the contract for another two years, and I'll need to start paying the $10 fee for the upgraded phone.

Since I have to renew the contract, will the second line also need to start paying the fee, or will it remain grandfathered for a $0 fee?

(Sorry for what's probably an obvious question, but I had trouble finding a solid answer so far.)

 

I didn't say what phone I had because I was too embarrassed: I haven't upgraded my Samsung Moment yet! Actually, I'm just kidding -- my Moment has been reasonably good compared to others' experiences around here. For example, the 2.1 upgrade gave me a huge battery life improvement. I'm a little gun-shy about going Samsung again, though. I'd still have upgraded to a new phone by now, but I decided I could wait a few extra months for the EVO3D or similar.

Thanks for the info! -- that's basically what I expected but didn't want to be surprised when I walked into the store in (crossing my fingers) a couple weeks. My plan is to upgrade just one line to the EVO3D right now; the second line we can wait on until the 2-yr date. I was just wondering if the mere fact that I was upgrading one line would give them a reason to charging me for all "premium data" lines on the contract.

("Grandfathered" probably isn't quite the perfect word here. I simply meant that had I bought them with a new contract after Sprint's changes January 30, Sprint would have charged the $10 monthly fee for both phones.)