You would like a process to be executed when a To Do is updated.
To Do does not appear in the list of Primary Objects, and when you select Task, the To Do subobject does not have the Previous Attribute Value to select.
The To Do object is not process enabled, so it cannot be used in the Auto Start condition as a primary object.
When used as a subobject, the previous value will not be shown: KB 200642 - Auto-start condition (Subobject - Previous Attribute Value) not available