Macro needed for changing cells to zero.

try so

Sub ertert()
Dim r As Range, dt#
With Application
    .ScreenUpdating = False: .Calculation = xlCalculationManual
    For Each r In Range("E9:CQ1010").SpecialCells(xlCellTypeFormulas)
        dt = r.Value
        If dt < 0 Then r.Formula = r.Formula & "+" & Replace(CStr(Abs(dt)), ",", ".")
    Next r
    .Calculation = xlCalculationAutomatic: .ScreenUpdating = True
End With
End Sub