Sprint and US Cellular do not have SIM cards so it doesn't have anything to do with SIM cards. IT has to do with the programming of the phone. You can use your US Cellular phones on Sprint but you would have to jail break them. I don't know how to do it. I believe it is fairly easy but I haven't ever had a need to learn.
If they sold your account you would also be subject to Sprint coverage as opposed to US Cellular. In the area I live in Sprint is seriously lacking.
If you are looking at leaving Sprint altogether once the transition is complete and you don't mind losing your 4G service look into prepaid. I know Walmart carries Straight Talk. You have to buy a phone (and they don't have the S3 only the S2 although I can't remember what network it runs off of. Either Sprint or AT&T). Straight Talk is $45 a month (per phone) for unlimited everything (talk, text and data.)
All the other major carriers have prepaid as well but Straight Talk was the absolute best deal. When I worked at Walmart (and I worked in the Electronics section) Straight Talk was the #1 top seller. However they sell Verizon, Sprint, Virgin Mobile (which has really good plans too if you want 4G) and a few others.
There are also some available online that aren't sold in major stores. Like Talk For Good (which I currently use.) So you can look around as well.
Bottom line is A you jailbreak your phone or B transition to Sprint or C get another carrier. If you don't jailbreak your phones you can either use them as a "mini-tablet" or sell them.
Hope that helps, let me know if you have any questions, I am a fountain of knowledge on most cell service. :)