Yes, File - Rename File is yet (UE v14.20.1.008) not available as script or macro command. So there is no other possibility then using a user tool. Save As is not good because it does not delete the original file and therefore is not a real replacement for a file rename operation.
You would need to save and close the active file, create a new file where to insert
"full path of just saved file" "new file name without path"
select that string and run the tool which has the command line:
Then you can close the temp file without saving it and open the file with the new name. Of course you don't get back with this method anything about success or failure of the rename done by Windows command ren. You would need to capture the output of the ren command and evaluate it with the script if you need to know also if the rename operation was successful.