From my own experience, yes the phone you purchased with your upgrade will be tied to your account. I purchased the iP4s with my upgrade on my second line, sold it on eBay, factory sealed. I didn't open it or do anything with the phone. Thankfully, the person who purchased it lived very close and we were able to get together and sort everything out. BUT, when they got the phone, they tried putting their sim (from their iP4) in to the new phone, but it kept asking to activate it under my line. IT WAS A BIG MESS!!! Too many details to get into here. It's made me reconsider ever doing that again, At least we were able to sort everything out with the help from AT&T and Apple who thankfully were both in the same mall. lol
Not sure why it got so screwed up. I've upgraded each time to the new iPhone that has come out and sold the previous iPhone on eBay (after wiping everything off of it and removing the sim) and I've never had any issues with the buyer having problems using it on their line.
I think the issue was that now there is an activation that has to happen (don't recall having to do that prior to the iP4s). If I had activated the phone, removed the sim prior to shipping it, then I probably wouldn't have had this issue, but the intent was to sell it factory-sealed. I believe it's Apple's way of making it more difficult to sell to the "grey" market.
iPhoNettie