From the FAQ:
I am currently with Sprint on a 2 year agreement that was started before August 22, 2014. Can I change plans to the Sprint $60 Unlimited Plan? In most cases, yes. If you have a multi-line account, and at least one line is off-contract, you are immediately eligible to migrate. If you have a single-line account, you must wait until you upgrade to a new device or your contract expires. You also can add a new line of service to your account to migrate. If you have additional questions, please call *2 or visit a store to work through the best option based on your needs.
Doesn't mention anything about an up to $25 a month charge. Just that if you are single line under contract you aren't switching to this plan.