I use Photoshop CS3, and though I spend a fair amount of time working with channels, I've always had trouble switching them around. I normally convert a channel into a layer before I begin working with it, just because I like the flexibility. I'm sometimes able to replace the old channel through Apply Image, and sometimes I can just copy and paste, but some of the time I am only able to create a new channel, which isn't useful at all. I don't understand why this doesn't work all of the time. I've found a few old threads about this, but none of the solutions offered have helped me. What am I doing wrong?

I copy stuff to and from channels on occasion, and havn't really seen a problem It might help if we had an idea of what you are doing when you run into the problem.

If you have the channels converted to greyscale layers, you can easily see the composite output without remounting the channels just by setting up the Blending Options.

For example: If your greyscale layer represents the Red channel, open up Blending Options, go to "Advanced Blending" and un-tick the G and B boxes. Now the Green and Blue information will come from the layers below and the Red channel of you greyscale layer will be inserted.

Of course, you can play around with the channels by ticking / un-ticking other boxes.

Hope this helps,

Rô