We are (newly) using civi to track membership in our international martial arts organization.
We have an external ID for each member to date, which I need to preserve and display in the contact summary, so we don't need to re-issue new member IDs.
Once I have the civi DB current with all existing members, moving forward we will simply assign NEW members the civiCRM ID as their member ID.
We had considered a one time re-issue of existing members a new ID based on their civiCRM ID, but this is not feasible.