UPDATE: I think the behaviour of "OldValue" is not correct
since, for example, when I insert a formula in a cell for the first time, OldValue should be NULL since the cell was initially empty, instead it contains the formula inserted!
See the attached screenshot (I used the custom formula "CAREA(2)" just for testing).
It means that the first time I insert a formula OldValue contains the formula self, instead when a recalculation is performed (because I changed something in another place of the sheet), OldValue contains the previously calculated value.
I think, in order to let OldValue have a coherent behaviour, when a new formula is inserted then OldValue should be "" or NULL (maybe NULL is more logical).
Instead if the cell already contains a formula, OldValue should ALWAYS contain the old formula result (this is not happening, in fact if I modify the formula in Old Value I get the formula I just inserted).
Thank you