For all windows 8.1/8/7 users it will be a free upgrade if they do it within 1 year of the launch. They will get further updates after one year and no u won't need to pay for it.
After 1 year if u try to upgrade to W10 from a W8.x device u will need to pay.
Most people think that free upgrade for 1 year means after the year end they need to pay or downgrade to old version but this isn't true.