As far as I'm aware, a data plan is a data plan. A data plan specifies how much data you are allotted per month, not how fast you download that data.
My phone is capable of 4G but if only 3G is available it still connects and works, only at the slower data rate.
I have an unlimited data plan that I've had for quite a few years. When my device was only 3G capable, my data plan worked then, when I upgraded to a 4G device, I didn't change my data plan. After Verizon dropped the unlimited data plan option, I upgraded my phone by buying it outright instead of upgrading thru Verizon and was able to keep my unlimited plan.
