Change between data validation listbox or formular in the same cell

[bosstec]

Hi,
I have the following problem:
Cell A1 have data validation listbox, where I can choose between values ("A", "B" or "C") from a named list called "List1".

Cell B1 contain normally a formular [=sum(D1:E1)], but when I chose the value "C" from the list in A1, I need B1 to be a data validation listbox as well, with another named list called "List2".
When A1 change to "A" or "B" again, I need the formular from before in B1.

In other words:
If A1 = "C" Then B1.DataValidation.List "=List2" Else B1="=sum(D1:E1)"

I was thinking the trigger should be Worksheet_Change

Thanks in advance