The wording of the OP is a little bit confusing when compared to the thread’s title, but I think that either of the potential questions can be answered for you pretty simply.
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The thread title seems to be asking whether you need to buy another Xbox One to be able to play KI again. As others have said the game is now available on PC (Windows 10 only), and whatever you purchased or owned content-wise will be available on either platform. If you do not log into KI on an Xbox One before logging into KI on a PC, the only thing to be aware of is that your game save will not be available, meaning your profile will start over at Level 1 with characters at Level 1 and nothing unlocked except for characters you own.
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The OP itself seems more concerned with playing “catch up” and learning characters and mechanics which were released after you sold your original console. That’s something that can certainly be done with time and commitment, and will be easier for you the better of a player you are in general. It will be more difficult than learning each character individually as they released would have been, but if you pick a couple of characters at a time to focus on you should be able to learn new content a little bit at a time. You can also check out the Academy or GG threads to find someone from the forums to play with and teach you specific characters.
Academy thread: http://forums.ultra-combo.com/t/killer-instinct-academy/
GGs Thread: http://forums.ultra-combo.com/t/the-ggs-thread/
The game is honestly stronger than it’s ever been, considering we’re in the third season of characters, have guaranteed tournament support for at least two years, and have a lot of new content and merchandise available or releasing in the future. I don’t think you have to worry about the community disappearing any time soon.