I have worked for a premium retailer for going on three years. I can confirm that for the past couple of weeks at least you can keep unlimited data regardless of whether you're using another line's upgrade or even your own upgrade (and therefore extending the contract ON the unlimited line) as long as you have another line on the account that doesn't have unlimited...doesn't matter if its a basic phone or a tiered smart phone, or anything else really. We need this because we have to have a target line.
This is pretty nice because before, while we could keep the unlimited as long as we used a different line's upgrade, now it's letting us actually use the upgrade on the unlimited line itself, which it did not do before. I have even called several friends who I knew had purposely not used their upgrade on their unlimited line yet and many have now used that upgrade successfully. After all, you never know if this is actually considered a loophole and will be fixed.
And no, this does not require scanning in a basic phone first, the only phone scanned is the one you're buying and it is listed on the customer agreement, so we should be getting paid accordingly.
If anyone has any questions feel free to PM me. Heck, maybe I could even make a sale or two and ship you a phone if you don't want to deal with having to make sure whatever store you go to does this correctly...you never know how long the person has worked there and if they know what is going on. For instance we have a new guy and I always make sure to assist him with these types of things as eRoes can be a pain in the ass to use sometimes. (if this is not allowed, then scratch that idea! lol). I have 500+ FB on ebay so could even handle it there for buyer peace of mind.