Early upgrade plans cost differently between each carrier. Some carriers charge more than others. Some carriers you can upgrade after 6 months, some 12 months and some even 18 months. It all depends on your carrier and the plan you've chosen.
Something else about Early Upgrade Plans, they basically go hand in hand with leasing a device. At the end of the contract, you will not own the device unless you purchase the device outright.
Here are all the iPhone 6s 16 GB Early Upgrade Plans on WhistleOut
Here are all the Samsung Galaxy s6 32 GB Early Upgrade Plans on WhistleOut
These results will allow you to compare data inclusions, minutes/messages and prices to choose the best plan.