Pierre, there's no way to convert one type to the other. However, if
there are many to change, you could use a macro to delete the existing
comboboxes, and add the new ones. For example, the following macro adds
comboboxes in column B, using a named range (MonthList) as the
ListFillRange.
Sub AddCombox()
Dim oleObj As OLEObject
Dim c As Range
Dim i As Integer
With ActiveSheet
'delete the comboboxes from the Forms toolbar
.DropDowns.Delete
'add comboboxes from Control toolbox
For i = 1 To 10
Set c = .Range("B" & i + 1)
With c
Set oleObj = .Parent.OLEObjects _
.Add(ClassType:="Forms.comboBox.1", _
Left:=.Left, Top:=.Top, Width:=.Width, _
Height:=.Height)
End With
With oleObj
.Name = "MyCombo" & i
.ListFillRange = "MonthList"
End With
Next i
End With
End Sub