For example: I want to update some proposals (since global changes for proposals are extremely limited). I create a new import. In the general tab I say "Create Import file" and include the fields I want to update and the proposal import ID. I can't include the constituent import ID. RE doesn't allow that. But when I go to update existing records, it tells me that the constituent Import ID is required for import.

Every Import type should be configured to allow you to export the required fields for an import of that type.