Migration from Bugzilla to Project Tracker is straightforward using a tool like PTMT. Migration from Bugzilla to Issue tracker depends on the version of Bugzilla. Recent versions of Bugzilla have "customizable fields" that are not present in the current version of Issue tracker. Hence it's likely that you’ll lose data if the migration is from Bugzilla to Issue tracker. If the version of Bugzilla and Issue tracker is the same, no data is lost. To perform a migration, please file a support ticket.