To be 100% clear -
Costco's policy is to allow you 90 days to return or exchange the phone. Period. Not just for the same model.
After 14 days, you will be tied to the plan you signed up for, but not the phone.
Wireless Associates (the ones that run the kiosks in Costco), will try and tell you that the phone can only be exchanged for the same model. This is false. The reason they do this is because they are penalized for returns, and an exchange for a different model counts as that.
One call to WA's central help line will put the WA associate back in their place regarding what the official policy is through WA and Costco corporate. The number is 1-888-369-5931. The manager who helped me when I exchanged the Charge for the Bionic (after about 80 days) is named Linda.
After about a 2 minute phone call with Linda, and her talking to the WA guy for about a minute, my exchange of the Charge for the Bionic was in progress.
And Meth - No. I did not have to pay full retail price on the Bionic. It was kept at the same "new subscriber" price.