If you've already done the upgrade, then you should be able to do so again later. SFAIK once you've already done the upgrade, the product is registered with the activation servers indefinitely and the one year limitation on initially upgrading is discarded. In fact, after upgrading, you can also do a clean install of Win10 the next time as long as you don't change motherboards.
If you change motherboards you will apparently have to go through the process of installing your 8.1 OS again, upgrade and then call Microsoft for reactivation.