You can sort of add the event you are looking for with two methods. The first one will add the the event whenever the user accesses the cell for editing:
Private Sub gvMast_EditingControlShowing(ByVal sender As Object, ByVal e As System.Windows.Forms.DataGridViewEditingControlShowingEventArgs) Handles gvMast.EditingControlShowing
Dim cboTemp As ComboBox = CType(e.Control, ComboBox)
RemoveHandler cboTemp.SelectionChangeCommitted, AddressOf StatusComboBox_SelectionChangeCommitted
AddHandler cboTemp.SelectionChangeCommitted, AddressOf StatusComboBox_SelectionChangeCommitted
End Sub
The second will perform the task you wanted to put in the selection change event and must have the same name as the AddressOf method you point to in the first one:
Private Sub StatusComboBox_SelectionChangeCommitted(ByVal sender As Object, ByVal e As EventArgs)
End Sub
---EDIT---
I forgot to mention that inside that second method you haven't yet changed the cell value...so if you want the old value of the cell you would get it like this:
Dim strCurrentMastStatus As String = gvMast.SelectedRows(0).Cells(strColumnName).Value
And to access the NEW value that the user has selected you would use this:
Dim cboTemp As ComboBox = CType(sender, ComboBox)
cboTemp.SelectedValue