You would really need to change the formula to do that:
=If(iserror(originalformula,"Open",OriginalFormula)
Various errors will be trapped like this.
If you didn't want to change all the formulae in the workbook then:
Edit, goTo, Special,Errors to select just the cells to change.
Macro recorder will convert that to VBA code.
For each c in selection
c.formula = "=If(iserror(" + c.formula + ",""Open""," + c.formula + ")"
Next c
Now that was untested but should get you started. If you have any problems on the VBA side then you should post in forum707.
Gavin