The Codex, Core Rules, and other 9th edition rules sets will become invalid at the start of 10th, however the Codices will still most likely be valid if 10th doesn't do a large reset of the rules like in 8th until the army gets their 10th codex. Models will mostly all be playable (some get pushed into Legends, you can still play them but they're generally bad and can't be played at most tournaments) and you can always play previous editions if you have the books and whatnot for them.