I need advice for a sim card situation!!

So here's the situation:

I currently have a Nextel i850, and it sucks.

It turns off, restarts, and I just plain don't like itt.
We (my family & I) have another year on the plan, and we don't plan on canceling, and we're switching services after our year is up. So that eliminates going and getting a new Sprint phone, because they told me it's a two year service agreement.

I found a Nextel i870 cellphone on ebay that I'm watching.

The question that I have would be:
Can I use my sim card from my i850 with the i870?
Also, would the phone have to be unlocked or could I just pop in the sim card, and have it work?

It would be great to receive some replies so I know what I should do.
I'm sick of my phone not working, or half way working, and I hate having it like this being a somewhat new driver, and being lost all the time and such.


Thank you.

~Kaitlyn

I would think that it would work. I upgraded before with an old sim and didn't have a problem. But that is not to be certain for your phone. Best of luck.