Copy content of channels into other ones

[rich2005]

Try:
Colors -> Components -> Decompose ...choose RGBA and to layers
that makes a new image.
Copy the Alpha layer Edit -> Copy
Go to the Red layer and Edit -> Paste As -> Paste As Floating Data
Anchor that "Floating layer"  Either in Layer dock or Layer -> Anchor floating Layer or Mask
Colors -> Components -> Recompose
Go back to the original image for the change.

A 40 second animation of that:  https://sendvid.com/esy5aj3v