If you "upgrade" by getting a new phone at a subsidized price, starting a new contract, you will lose unlimited on the upgraded line. Doesn't matter what you upgrade from or to.
So you can pay full retail, get the phone from somewhere else, or do some line swapping/adding on family plans to keep unlimited.
I am due for an upgrade in a few months and I am really leaning towards the new iphone. My question is...I have an unlimited family data plan, if I switch to an iphone will I be forced to change data plans? I work in litigation and I use a lot of data. My bill is already outrageous and I don't want to have to pay their ridiculous data charges on top of the $30 "smartphone data plan" charges for both phones.
If you have an old calling plan, you may not have much of a choice but to change plans. If your plan is relatively new, as mentioned above, you can keep your unlimited data by paying full retail price for your phone, getting the phone from somewhere like ebay (Best Buy and the like will also result in the loss of your unlimited data), or try swapping/adding lines. Also, if you aren't aware, as of this spring there is a $30 upgrade fee.