Saving AT&T grandfathered unlimited data plan
I am moving to T-Mobile, and am selling my grandfathered unlimited data plan for my iPhone with AT&T. However, I'd like to keep my old number. I've read that some people can port over the old number while simultaneously changing their phone number with AT&T. The end result is you have your original number with T-Mobile, and you have a new number on your grandfathered plan with AT&T that you can sell.
Is this still possible? Does anyone have a success or failure story of you or someone you know? What are the details for how it's done?