String Tag Sequence

I need to use a String Tag that cycles through the letters of the Alphabet.

Thus if I have a two letter tag Starting AA it will loop through to AZ and then go onto BA and so on.

I have come up with the following code which does the job.

Can anyone suggest any better ways I might accomplish this?

VB Code:

Imports System.Text
Module Module1
    Function IsRestOfStr(ByVal Tag As String, ByVal Position As Integer) As Boolean
        ' Checks to see if the rest of the string is all Z's
        IsRestOfStr = False
        Dim StrTag As New StringBuilder("")
        StrTag.Append("Z"c, Tag.Length - (Position + 1))
        IsRestOfStr = (StrTag.ToString = Tag.Substring(Position + 1))
    End Function
    Sub ChangeLetter(ByRef Tag As String, ByVal Position As Integer)
        ' Changes the letter at the specified position
        ' Moves to the next letter in the sequence, if it is Z then resets it to A
        Dim SubStr As String
        SubStr = Tag.Substring(Position, 1)
        If SubStr = "Z" Then
            SubStr = "A"
        Else
            SubStr = Chr(Asc(SubStr) + 1)
        End If
        Tag = Tag.Remove(Position, 1)
        Tag = Tag.Insert(Position, SubStr)
    End Sub
    Sub NextLetter(ByRef Tag As String, ByVal Position As Integer)
        ' Checks the letter at the specified position
        Dim SubStr As String
        If Position = (Tag.Length - 1) Then
            ' If this is the last letter in the string then set to the next letter
            ' If it is Z then set to A
            ChangeLetter(Tag, Position)
        Else
            ' Check to see if the rest of the string is all Z's
            If IsRestOfStr(Tag, Position) Then
                ' If they are then increment this letter
                ChangeLetter(Tag, Position)
            End If
        End If
    End Sub
    Sub NextTag(ByRef Tag As String)
        ' Moves the letters in the Tag String to the next in sequence
        ' Runs through the entire string.
        Dim Position As Integer
        For Position = 0 To (Tag.Length - 1)
            NextLetter(Tag, Position)
        Next ' Position = 0 to (Tag.length-1)
    End Sub ' NextTag
    Sub Main()
        ' testing the code
        Dim StartTag As String = "AAA"
        Dim Idx As Integer
        For Idx = 1 To 680
            Console.WriteLine(Idx & " " & StartTag)
            NextTag(StartTag)
        Next ' Idx
        Console.WriteLine("Press Enter to Continue")
        Console.ReadLine()
    End Sub
 
End Module