B.D.
I have bought used phones from eBay before, popped in my SIM card and kept on a going. If you're changing phones you just need to make sure the SIMs are compatible.
I am not sure if this is your concern or not, but do you not want to be forced to change your plan? We were Nextel customers that got absorbed by Sprint. We still have our 8+ yo cell plan and are grandfathered until we give it up. It is considerably cheaper than the plans they have out there now, but it does have many limitations of things that just didn't exist back then.
A couple years ago we did get new cell phones (the old ones weren't capable of texting!). We did have to sign a 2 year contract but it was for the same plan with the same terms as before.
HTH