I would imagine it is unlikely as this is a permanent sale of the OS. They usually do discounted upgrades to a new version for those that bought near enough to the new version coming out so probably you'll be able to get it for not too much. But one of the big things about Windows 10 that people seem to be excited about (don't know why) is that it's supposed to bring more focus back to traditional form-factors and as this is a tablet, Windows 8.1 seems fine on it to me (not that I don't think 8.1 is fine on Desktop, too). The other thing is that I think Windows 10 is going to be available under a rental model too, so I don't see the appeal of that for a cheap tablet, tbh.
Anyway, cheap upgrade probably. Free, unlikely.