Hey Bill,
I'm not sure about ATT, but Verizon offers all your devices to share the same data plan, and if you want that plan, you'd loose your grandfathers rate. The iPhone can work as a personal hotspot for any wifi device in range, so if you don't need the iPad connected all the time - that may be a way to go. I use it that way sometimes. The only downside to that is that a call or text will interrupt the data and you'll get bumped.