how far are you from your upgrade? because if your eligibility for upgrade is close, i would suggest just waiting it out that way you'll keep you grandfathered plan since unlimited data is hardly available these days. companies that provide unlimited data plans have pretty slow connectivity so its like having the dial-up of data plans. but if you're keen on buying a new phone, might i suggest looking in your local area on craigslist first? there's always someone that uses their upgrades to sell a new phone at a cheaper price than retail to make some quick cash.
i wouldn't advise on going non smartphone, just from the trend of phone companies consistently releasing nothing but smartphones. i've had a few friends as well that switched to non smartphones in order to save money and eventually, they switch back. only people that i really see that are able to use regular phones these days are the elderly who don't care much about any other feature but being able to call people.
the sim card is transferable so you wont have any issue in that matter. only phone ive come across so far that's different is the iphone 5 which uses a mini sim i believe?
hope all that helps :]