Is it possible to replace one channel of 5:1 AC3 file without re-encoding its other channels?
Is it even possible to work with AC3 channels without re-encoding?
Since AC3 makes heavy use of channel coupling this might only be possible under very specific circumstances, if at all. At least you need an in depth analysis of your file to find out which channels are coupled. I’m not aware of any software tools though.
I'd guess that would mean something like bitstream-editing, and you need to provide that channel that you would like to replace in exactly the format that it fits in. And there is, as already mentioned, the channel coupling, and last but not least the CRC checksums. So, in short terms: it is not really possible ! Decoding, replacing one channel and re-encoding is what you will have to do.