The best, most active a/c discussion forum is at www.aircondition.com. They mention 406, but it doesn't seem to be very widely accepted. There is much more known about 134. If you're going to change, 134 is relatively cheap and there is a good knowledge base of information to draw from. If you try 406, you are out there flying around on your own. At least at this point you are.

Good luck,