It seems like it is not possible to retrieve the values of the previous version of an entity. Only the values that have actually been updated can be retrieved - the rest are uninitialized. I attach a test case to prove my point.
I've used the SaveFieldManager in a @PreUpdate callback in an attempt to store a serialized version of the previous version of the entity whenever the entity is updated. I do not only want to store the changed values but the whole entity.
This functionality must be available and also possible to achieve in a supported audit log.