You can do a swap when you do the phone purchase. But like I said, I'd do it at Best Buy or somewhere that does instant rebates. Maybe it has recently changed, but in the past 10 years, whenever there is a mail-in rebate with Sprint, the phone has to be activated on the line that had the upgrade on it for 30 days or the rebate becomes invalid. I got around that once when I didnt understand how that worked, but it was a hassle and had to hound them when the rebate got denied.
So like I said, if you have any line that is eligible for the upgrade discount, you can go into Best Buy, purchase a phone using the upgrade discount and then they will immediately switch it to another line on your account of your choosing, and in that case their rebate is instant so you arent dealing with a mail-in rebate. If you do that directly with Sprint, I'd read the terms of the rebate very carefully, because doing that with a mail-in rebate has always invalidated the rebate.
I've been with Sprint for almost 11 years now and my advice is if the line you are upgrading isnt the one that is actually due the upgrade, just go to Best Buy or radio shack where the rebate is instant. If the line you are upgrading is the one that is due the upgrade, you can just go through Sprint and do the mail-in rebate if you like.