If you bought discounted Pro licenses for 8.1/8 when it came out (I paid $35 AUD each) they do validate and install windows 10 Pro - tested literally 1 weeks ago.
Yeah, it did that for me and it was a seamless transition. However, MS did announce that updating to Win 10 from 7 or 8/8.1 would cease, and as far as I know, it did. Thing is, MS came up with other ways to update for free. assistive technologies being one such scheme, and I have a feeling that these free updates using 7 and 8/8.1 product keys were quietly reintroduced to get more people onto 10. I say quietly because I think MS didn't want to be seen as going back on the 'no longer free' statement.
I don't know why anyone would like windows 8.1 to my opinion it is worst than Vista. Strike 4 for windows on that one.
Not everyone's cup of tea, Win 8.1, but once I had Start8 and other Stardock stuff on it, I thought it was actually better than win 7. It was definitely faster, and once I got rid of those confounded tiles, it looked better, too. Apps and tiles belong on phones, not a PC.
And same as ALMonty.... never had a problem with it, which is more than I can say for 10. In the early days, 10 was like an unruly child with attitude, though I must admit, it has much improved since then and behaves so much better.