How to loop a Subfolder in a Function GetFolderName

Question

I've a macro to convert the excel files to different format ( Mostly to convert it form the xls to xlsx)

however it does not run through the sub-folders where ive more then 2000 files in subfolders.

How do I add a loop to make it run through a Sub folder ?

    Private Type BROWSEINFO ' used by the function GetFolderName
    hOwner As Long
    pidlRoot As Long
    pszDisplayName As String
    lpszTitle As String
    ulFlags As Long
    lpfn As Long
    lParam As Long
    iImage As Long
    End Type
    Private Declare Function SHGetPathFromIDList Lib "shell32.dll" _
    Alias "SHGetPathFromIDListA" (ByVal pidl As Long, ByVal pszPath As String) As Long
Private Declare Function SHBrowseForFolder Lib "shell32.dll" _
    Alias "SHBrowseForFolderA" (lpBrowseInfo As BROWSEINFO) As Long

Function GetFolderName(Msg As String) As String
' returns the name of the folder selected by the user
Dim bInfo As BROWSEINFO, path As String, r As Long
Dim X As Long, pos As Integer
    bInfo.pidlRoot = 0& ' Root folder = Desktop
    If IsMissing(Msg) Then
        bInfo.lpszTitle = "Select a folder."
        ' the dialog title
    Else
        bInfo.lpszTitle = Msg ' the dialog title
    End If
    bInfo.ulFlags = &H1 ' Type of directory to return
    X = SHBrowseForFolder(bInfo) ' display the dialog
    ' Parse the result
    path = Space$(512)
    r = SHGetPathFromIDList(ByVal X, ByVal path)
    If r Then
        pos = InStr(path, Chr$(0))
        GetFolderName = Left(path, pos - 1)
    Else
        GetFolderName = ""
    End If
End Function

Public Function FileFolderExists(strFullPath As String) As Boolean

'Macro Purpose: Check if a file or folder exists

    On Error GoTo EarlyExit
    If Not (Dir(strFullPath, vbDirectory) = vbNullString) Then FileFolderExists = True

EarlyExit:
    On Error GoTo 0

End Function