Its a little long, but here is one way:
Copy this into a module:
Declare Function apiCopyFile Lib "kernel32" Alias "CopyFileA" _
(ByVal lpExistingFileName As String, _
ByVal lpNewFileName As String, _
ByVal bFailIfExists As Long) As Long
In your form call:
Dim sFilePath as string
dim sNewFile as string
sfilepath = 'path and file name of .txt file
snewFile = Replace(sfilepath,"txt", "csv"

apiCopyFile sfilepath ,sNewFile, False
'delete file from current directory
Kill sfilepath