I'd like to know if something will glitch out or change drastically when the patch hits. I don't want to keep playing on a "doomed" world, and it's my first transition between patches where I want to keep the same world. Specifically, I'm afraid of what will happen in the End.
There hasn't been anything that would "glitch out" your world since they changed saving format to anvil(in beta). So everything will be fine.
With The End: When you launch the world in 1.9 for the first time it will ask you if you want to reset it.
If it doesn't you can craft Ender Crystals(Not 100% sure how to use them but it will tell you). The Ender Crystals will respawn the Enderdragon and by killing it all the usuall stuff will happen.(I don't think this will reset the end world but the dragon can break anything built with anything other than endstone or obsidian)
We've got a fairly nice Enderman farm, but I'm leaning toward resetting the End with 1.9 anyway, and will probably build an enderman farm near the outer edge of the void between the center island and the 'ring'. That way players can use the pearlporter to get back and forth to it without having to worry about the bridge (to the original Ender Ender from the main island) taking damage from subsequent dragon fights.
The End will be entirely deleted if you choose the reset, but if you don't and simply resummon the dragon I believe the only thing that happens is the old portal gets erased (the portal gets turned off and the bedrock gets replaced with endstone) until you kill the dragon again. That said, if you had significant building more than 800 blocks away from the mainland (I believe it was stated the gateway islands form at about 1000 but I'm including a buffer) I dunno if the game would overwrite it with the islands or choose to skip over the way that ocean monuments were affected in then-existing worlds. For all I know you might end up with some floating chorus forests or end cities.
With the overworld and nether, there shouldn't be much changes that occur. You may get an abrupt border/biome change in some newer areas that are next to your generated areas, but besides that it should be stable.
The end dimension though might be affected. Im not too sure if the new areas will generate it without resetting the end, but thats about it.