Help With Duplicate Account Numbers

Hey

I currently have this code which tells the user there is a an accout number already on the db:

Private Sub AccountNo_AfterUpdate()
Dim rs As Recordset
Set rs = CurrentDb.OpenRecordset("Select * From Spreadsheet where AccountNo = '" & Me.AccountNo & "';")
If rs.RecordCount > 0 Then
MsgBox "This Account Number Is Currently Within The Database And Will Not Be Allowed.", vbInformation, "DD Cancellations"
Me.AccountNo = Null
End If
End Sub


I want a peice of code which instead allocates a letter to the duplicate account, so user enters a duplicate account, the account will then turn into 123456789A, upto 5 duplicates are allowed, so upto 123456789E. Once "E" has been reached accountnum box will turn null with msgbox. I tried out Case statements but that doesn't seem to work.

all help appreciated. Thanks.