With the update of horses I have a problem; I can't find any. Do you need to make a new world for the update or do all of my other already managed worlds automatically get updated with horses?
What I've seen says that they will spawn in existing chunks, but only if you kill everything in that chunk.
This is because there is a limit on the number of passive mobs in a chunk, and that limit is very quickly reached. By killing everything in an area, new passive mobs will start to spawn to fill the cap again, and some of them might be horses.
Yes, horses will spawn in old worlds (answer to question). I made my world in 1.5. I'm using 1.6.2 and I have found horses just by simply 'looking harder and further'. Killing everything in the chunk does not make mobs spawn necessarily. If you travel around 1000 blocks in a random direction you WILL find a small amount of horses. Travel further and you will find more horses. Horses are rare, so you just have to keep looking.