All, I just wanted to post this as I just successfully did this, with a lot of frustration along the way.
To dispel any rumors, US Cellular uses the CDMA network, NOT GSM. If you are going from Verizon to US Cellular, you don't even need to unlock the phone, go into the store and they will insert a US Cellular SIM card in it and you'll be on your way.
US Cellular to Verizon is a little more tricky because Verizon's network is a little more locked down. Before you purchase your iphone, get the IMEI number (or a screenshot of the settings screen to be on the safe side) and call the US Cellular corporate number to verify that the phone is not reported lost/stolen and that it has been completely removed from whatever account it was tied to. Then once you purchase the phone, contact the US Cellular corporate office to request the phone be unlocked, which they will do free of charge and will normally take two business days. I would then remove the US Cellular SIM card BEFORE walking into the Verizon store and ask them to activate it, which they will provide a Verizon LTE SIM card for you.
I made the mistake of telling the guy at the local Verizon store that it was a US Cellular phone and after I adamantly asked him to insert a SIM card to try and activate the phone, he said it was impossible and said all he could do is give me the trade in value of the phone on a Verizon gift card. Frustrated I called the Verizon corporate office up and the woman I spoke with was nice enough to send me a SIM card through the mail and waive the 2 day business priority shipping. Once I received it and waited the two days for the phone to be unlocked, I simply called the Verizon corporate customer service number again and asked them to activate the phone (didn't mention that it was previously on US Cellular) and it went through just fine. US Cellular recently signed an agreement to become part of the group of major us cellulars that will willingly unlock their phones for you, even if you're not a customer. Other companies overseas charge a lot of money for this service, so DO NOT BE FOOLED.
Also, if someone tells you this isn't possible, hang up or walk out and speak to someone else as I'm proof that with a little footwork this can be accomplished.
Also if you look on eBay or Amazon, US Cellular iPhone 5s/6/6+'s are usually cheaper than their Verizon counterpart, even though they are the EXACT same phone.
Hope this helps someone...