I mean, if you feel like paying for it, you can definately get an old Talk & Text plan and add a line while keeping your unlimited data...but the price you would be paying would be more than the unsubsidised price of the phone.

One think you might try is getting an HPC device and activate it as a Month to Month on your account, its only $20 a month (also cool if you already have home phone service). Use the upgrade on that month to month line, get a new phone, activate it on your unlimited data line and reactivate your HPC line.

That option will cost $400 for 20 months, but you will always have an upgrade that you can burn for new phones every 20 months.