試しにマクロを組んでみました。
検証が不十分ですが、ほぼ、いけてるかも知れません。
数式内に、ROUND関数が1個のみに限定です。
Sub test1()
Dim c As Range
Dim cf As String
Dim hf As String
Dim kf As String
Dim myflg As Boolean
Dim i As Long
Dim j As Long
Dim n As Integer
Dim k As Integer
kf = "ROUND"
For Each c In Range("A1:A10")
k = 0
myflg = False
cf = c.Formula
For i = 0 To UBound(Split(cf, "("))
If InStr(Split(cf, "(")(i), kf) Then
cf = Replace(cf, kf & "(", "")
n = UBound(Split(cf, ")"))
For j = n To 0 Step -1
If k = i Then
hf = Replace(cf, "," & Split(Split(cf, ")")(j - 1), ",")(1) & ")", "")
c.Formula = hf
myflg = True
Exit For
End If
k = k + 1
Next j
End If
If myflg = True Then Exit For
Next i
Next
End Sub