There are no problems with backwards compatibiliy, since v0.6 hasn't change anthing beside the handshake! New v0.6 is just another handshaking -nothing more-.... assuming you have a good handshake parser it will add 2 lines more code to be still v0.4 compatible. Everything else is the same, in v0.6 you will use the same v0.4 Gnutella descriptors/messages.