I don't know what would happen if you try to replace a published channel with a pkg signed with a different key other than even if it is possible, you'd lose everything stored in the registry.
I assume you have access to the developer account that created the channel in the first place. If so, you can download the published signed package. It isn't clear to me whether you have the original password that goes with the original package, but if you do you can use the "Rekey" option under "Utilities" using the downloaded package. Once you rekey your box, any package you now create (using the original password) will be signed with the original key.
If you don't have that original password, then I think you're out of luck.
Hope this helps!
-JT
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