Usually we take care that your world still works when bringing out an update. It may still be possible that very large updates will break your world (we can't guarantee that it's always possible to convert the world, but we do our best to keep worlds working).
Yes, you can move your world to a server or share it with your friends. It is saved in the "Worlds" folder in your game directory. Usually the file size of worlds is ultra small (except when using lots of custom images).