My old world has been reduced from the normal 256 by 256 to around 50 by 50. This changed with the update as yesterday I was just on it. The blocks are still there, but inaccessible. When I make the world infinite, my old world reappears as is, with the additions of infinite as well. However, I would rather have the old format back as it slows down my phone.

The first image shows how it is, and the second shows how it should be, as when made infinite.