SMP server will not allow the use of a pre-existing map when changing over from a .jar to a .exe (since that's all that was released in the Development Versions wiki page). Managed to find an .exe and continued to have the same problem.

What I expected to happen was...:
To be able to continue my same map with the same players, only with the ability to use horses, leashes, etc

What actually happened was...:
Internal server error, [SEVERE] End of stream.

Steps to Reproduce:
1. Have an SMP server utilizing the Minecraft_Server.exe and an existing world file
2. delete old .exe and add new .jar/.exe
3. Run new file
4. Attempt to join server.

Client says this:
Client> java.io.FileNotFoundException: http://assets.minecraft.net/1_6_has_been_released.flag
Client> at sun.net.www.protocol.http.HttpURLConnection.getInputStream(Unknown Source)
Client> at kq.a(SourceFile:237)
Client> at bhq.run(SourceFile:114)
Client> at java.lang.Thread.run(Unknown Source)
Client> 2013-04-25 12:15:13 [CLIENT] [INFO] Connecting to 192.168.1.3, 25565
Client> java.io.FileNotFoundException: http://assets.minecraft.net/1_6_has_been_released.flag
Client> at sun.net.www.protocol.http.HttpURLConnection.getInputStream(Unknown Source)
Client> at kq.a(SourceFile:237)
Client> at bhq.run(SourceFile:114)
Client> at java.lang.Thread.run(Unknown Source)

and server says this:
[INFO] Atlus789[/192.168.1.2:54723] logged in with entity id 171 at (-895.8931137555869, 80.0, 572.9646343935808)
[INFO] Disconnecting Atlus789 [/192.168.1.2:54723]: Internal server error
[WARNING] Failed to handle packet for Atlus789 [/192.168.1.2:54723]: u: Sending packet
u: Sending packet
at jj.b(SourceFile:443)
at gw.a(SourceFile:122)
at gw.a(SourceFile:77)
at jh.d(SourceFile:161)
at jh.c(SourceFile:53)
at ih.a(SourceFile:44)
at ig.b(SourceFile:29)
at net.minecraft.server.MinecraftServer.r(SourceFile:488)
at ib.r(SourceFile:258)
at net.minecraft.server.MinecraftServer.q(SourceFile:405)
at net.minecraft.server.MinecraftServer.run(SourceFile:339)
at gr.run(SourceFile:582)
Caused by: java.lang.NullPointerException
at fh.a(SourceFile:66)
at ck.a(SourceFile:159)
at jj.b(SourceFile:426)
... 11 more
[SEVERE] Reached end of stream