When a unit upgrades, it is still the same unit as before. A new unit isn't created, the old unit simply has its type changed. You can do what you want by storing a unit's type whenever it enters the playable map area. When it upgrades, you have access to the old type, and then you can update it to the type it just upgrade to (for future upgrades).
You can't change the point value of a unit, so its point value will always be the point value of its type.
You could also save the type of a unit when the "unit begins an upgrade" event occurs.