Hey there
We can certainly help with transferring your balance to your postpaid account when you convert your line! For more information on now to convert the line, please click here. The purchase of a new device may depend on what payment option you choose. For example, some device purchases may require a payment to be made up-front at the time of sale, or in the case of AT&T Next, the only payment due at that time would be the sales tax for the device. Please also keep in mind that a security deposit may be required, and there will be a $40 activation fee for your new postpaid line.
I hope this helps!
Charise