Currently there is no way to track the reassignment of an assignment from one user to another.
Neither is there any way to track the assignment number before upgrade and link it to the assignment number after upgrade.
Currently a change to any assignment size is tracked in the audit log, but the above two are not.
A suggestion for tracking these items is here: Assignment Profile Page
Alternatively they might be added to the audit log, though the upgrades especially could bloat the log significantly.