When converting a Target to a Lead, all the history is lost – calls, meetings, emails, etc.
This seems to be a bug that has been known since at least SugarCRM CE 6.0 but was never fixed:
http://forums.sugarcrm.com/f3/converting-target-lead-missing-information-71842/
http://forums.sugarcrm.com/f3/lost-activities-afters-targets-conversion-69587/
In the meantime, suggested workarounds would be appreciated