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.
Download Minecraft 1.6.3 Pre-release:
Cross-platform jar: Download from Server 1
Windows EXE: Download from Server 1