So my customer says when he mutes his Denon receiver it changes his cable box to channel 001. I said it's pretty impossible but let me see. So I get there and sure enough it does it. If you mute it VERY lightly it works 99% of the time but if you press it hard it goes to channel 001!! And if you ramp the volume up or down the picture slightly distorts and occasionally goes to 001. The remote that is "causing" this is an MX450. I get the oem Denon and no issues. I use the cable company remote with a Denon code and it works fine too. So it seems that URC Denon code 0811 has been learned with some garbage in it. It was a little late so I though I'd "learn" it over the preprogrammed code, but I don't believe this can be done, can it? I didn't have time to check other Denon codes on the 450 last night. Here is a question: If I do replace the code with another (? delete 0811 first) will the original codes in my macros be gone? I'm almost sure this is what will happen, confirmation on this?

The mute button obviously has a programming conflict in it.    Just re-learn the mute button from the Denon Receiver to the Mute button on the 450.  It should take about 10 seconds.

The volume also seemed to affect the picture. My question now is: Can I "learn: a command over a preprpogrammed command? I guess I'll get on the phone with URC to see as well.

Yes, you can learn it over the existing command without redoing the entire remote.    

It sounds like you're having an issue with your "Punch Throughs" now that you've said that the volume isn't working properly either.  

Go in the remote for VOLUME and change the punch through to the receiver.  That should cure your problem.  

Or just relearn them.

I think these commands would show the problem whether they were punched through or not.

Lots of experience with Prontos, where it's easy to learn the commands and the editor lets you see the hex version of the command, has shown that remote control codes often come out differently with different learns, sometimes wildly differently. It's amazing to me that this kind of crossover problem doesn't happen more often. I"ve only heard of it three or four times in ten years. I loved the Sony volume up command that put the cable box on channel 52!

You can predict that there will be differences in the learns if differentl learns are done with low batteries, then fresh; if the background lighting is dark, then bright; if you vary the amount of time you hold down the button on the remote you're learning from... and just in general. Learns are rarely perfect and usually vary slightly.

Here, it looks like the two commands are close enough that your particular imperfect learns work both.