You can, but there are some exceptions that you should be aware of. first if you have a mail in rebate you will have to leave the new phone active on their number for 6-8 weeks while the rebate's being processed. when you swap the phone from phone number that you got the discount on, to your line you may have to factory reset the phone in order to complete the activation, and you may lose voicemail in the process. you should be able to upgrade line 2 at bestbuy, amazon, sprint, Etc. anybody qualified to offer sprint upgrades. please let me know if you have any other questions.
I'm currently in a family plan and my line (line1) isn't eligible for an upgrade discount until next year, Aug 2013. However line2 is eligible for an upgrade discount on Aug 2012, can I use the upgrade eligiblity on line2 towards buying a new phone for line1?
If yes, can I do this online with third party retailers like amazon or wirefly?