Need to transfer the PIX 501 config to a PIX 515…

So I have a PIX 501 that is questionable as far as reliability. I have been sucessfully able to backup the config to a text file on a tftp server. But when I go to restore the config to a newer 515 it seems that it doesnt see that backup as a PIX image file.

Does anyone know if this is even possible? I would imagine that it would work but have been successful at it. Im just looking to move this config to the other pix. I tried copy tftp flash and filled in all appropriate info but it errors out on copy.