It is as backwards compatible to v0.4, as v0.6 is to v0.4.

Protocol v0.7 proposal rev1 is full binary transparent with other/older clients and interacts with them without any problem. E.g. features like GUID tagging, ISO charset and optional UNICODE are full compatible even with v0.4, 0.5. 0.6 and 0.7 clients. To step further I think also features like hashs, metadata, tunneling and superpeers should be fixed part of a new v0.7 protocol (I prepared as much as possible for those proposals, see GUID tagging). See the v0.7 protocol as a possible replacement for v0.6. The intention/goal in my eyes is a more standardized and better documented protocol -> a step further to a Gnutella RFC.

I did also describe a upwards compatibility to a theoretical coming v0.8 protocol (see Appendix D).