previous_target for on_marked_for_upgrade

[_CodeGreen]

I've been trying to figure out how to fix a certain bug with redo, which involves upgrading the same entity twice in a row. The first upgrade works fine, but the second upgrade is causing some trouble. on_marked_for_upgrade happens after the entity was marked, which makes sense, but there's no way for me to tell what the entity was supposed to be upgraded into before giving it a new upgrade. An on_pre_marked_for_upgrade event is not necessary, but as the title says, an event data field like previous_target would completely solve that issue.