They won't let you just transfer a regular Sprint plan to SERO if you are under contract and keep your number. If you are under contract they will do it but charge you early termination fees and give you a new number. The trick is to open a new SERO account. Then you port your number from your old Sprint account to a prepaid phone.
After you open your SERO account and it is working you can then port your number back from the prepaid to your new "SERO" account.
Since you are under contract, you should just email them and ask for freebies or "perks" rather than going through all the mess. Go here for more info:
http://forums.slickdeals.net/showthread.php?t=563252