Just make sure that phone is actually going to end up in the right place.
A local outfit was advertising just such a deal. Upgrade and your old phone would be reprogrammed and given to a women's shelter. Turns out a bunch of them ended up else where without the reprogram. And it is possible, to have two ESN #s on the same phone service. They can use your plan to make calls. Some folks will turn a good thing bad.
I plan to drill holes in any old phones I turn in.

 

i don't know of a phone system currently operating where they will allow you to have two esns on the same phone system.