How do you access CBS All Access through Apple TV Channels if already subscribed

You don’t. Your existing subscription is not exchangeable with the Apple TV app Channel subscription. Either use the CBS subscription with the CBS app and/or web access (if supported), or cancel that and go for the subscription within the Apple TV app.

Argh! The irony for my is that I purchased my CBS All Access subscription on my Apple TV device and thus am paying for it through Apple. I wonder why Apple doesn't make "simply work" in this case and recognize that in the desktop Apple TV app.

Purchasing the CBS All Access subscription on your Apple TV device and paying through iTunes/Apple (but not initiated through the Apple TV app), is still a different subscription than the Apple TV app CBS channel subscription. You don’t have to switch; you can keep using the existing subscription with its own app and/or web access.

Making it “simply work” would not be technically difficult, but the licensing requires a separate subscription.