might have to go with atmosphere. it works on the latest with sx core, I believe. you'll lose some features in the process, but who knows if tx will ever update it. you can probably go back to it if they do. though, I've only seen a few updates that required the latest firmware, and those can probably be downgraded. they haven't updated the keys in a long time, so you'll probably be good for a while.
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it might also depend on sdk used whether you can downgrade the game. I know 3d world is playable on 10.0.0, but something like super meat boy requires 11.0.0 with it's update requiring the latest (dunno if it can be downgraded).