Update: Minecraft pre-release of 1.6.3. You can select these versions in your launcher, or download the server links below.
We still strongly recommend that you don’t run the snapshots on an existing world if you don’t want to run into chunk errors. There is a known issue with world structures such as Nether Fortresses not transitioning to 1.7 from 1.6, but loading these areas in the new pre-release 1.6.3 and then going to snapshot 13w37b (or later) should fix this.
To get the pre-release, do the same but manually select “1.6.3? from the version dropdown list.
Credits: The Minecraft and Minecraft Realms teams