I believe people form discourse communities to accomplish a common goal with like-minded individuals. People join discourse communities to be around those same like-minded people or they truly believe in that groups cause.
Rules, policies, unique writing, and lexis are necessary to distinguish that discourse community from the general public or other discourses. These policies and rules help give the discourse its identity so it is vital that they exist.
Writing is key as it is more tangible. Writing can be referred back to for future members of this discourse. Speech can be documented but not in its entirety. Speech can of course inspire and motivate but it does not build upon itself the way writing does.
Changes in discourse communities can happen, but the rate of change can be equal to the urgency of the situation. Changing the goals of a traditional long-standing discourse community may be impossible, while Changes to a newer or less traditional discourse may be much easier. Changing the goals of a discourse community would have to start from the top and trickle down through the appropriate discourse channels. If accepted by the entire community, then the goals have been effectively changed.
Jeffs Blog 2.0 