How do I change the contribution type for a contribution that was made before 1/1/1996?
Date Preferences at civicrm/admin/setting/preferences/date?reset=1 can be edited so the 'offset' is greater than 20 (years)

SO you can edit Date for activities (including contributions: receive, receipt, cancel. membership: join, start, renew. case: start, end.) and set it to have years going back further than 1996 (ie 20 years)