In considering migrating to a new platform, what considerations should be made in regard to existing content? This move is needed to due various reasons (pick something...needed features, integration with the outside world, host migration that doesn't support the existing platform, etc.)
As an example, when migrating from one forum software to another a community has multiple options on how to deal with old content. One option is to migrate it all content (posts, users, groups, private messages, etc) to the new platform. This is not entirely reliable as my experience has shown that user groups and permissions rarely migrate successfully if you have somewhat complicated permissions. This leaves the brand new forum in a semi-functional state.
Another option is to close the old board and prevent new posts and members. Leave a link to this place as an archive, but force new content to be created on the new board. The disadvantage here (from a user's perspective) is that everyone is starting fresh. Members that have years in the community suddenly have post counts equal to that one guy that registered yesterday.
Obviously the community doesn't want to lose this old content, but they don't want to migrate to something that doesn't work either. How can a migration be handled successfully without losing existing data?