Sprint SERO Question

[steps]

I just got my SERO phone today and I want to switch over my Mogul to the new number and wanted to know how long do I have to wait before I can switch over my Mogul to the SERO plan? Meaning I want to "swap phones" I have a regular Sprint account (which I pay $100+ a month) and was able to sign up for a new line with the SERO plan. I want to switch the phones so the Mogul is under SERO and the "new" phone is under the old sprint account. Is there a waiting period?

edit:
am I even able to swap the phones or will I have to get a "dummy" third phone from ebay.



Thanks!