According to the MicroSoft web page, they state:
Is the upgrade really free?
Yes, it’s free. This is a full version of Windows, not a trial or introductory version. It is available for a limited time: you have one year from the time Windows 10 is available to take advantage of this offer.
Indicating you will never be asked to pay for it. Of course, any new computer you purchase will require the purchase of the OS on the machine you purchase. No different than now.
I am not sure I am going to rush into the upgrade, there is time to watch, wait and see how it goes.
W.