Username and passwords are not stored in the rdp file.
They are stored somewhere else on your OS, on Windows vista or above they are stored on cmdkey
To view your current credentials go to command prompt and type cmdkey /list
From the link you can learn how to delete them as needed.
From your script you will first need to delete the credential for the rdp server then add the new one.
cmdkey /delete:rdpserver.domain.com
then
cmdkey /add:rdpserver.domain.com /user:Kate /pass:passme
OR add the credentials then delete at the end of your script