I am afraid once you credit miles somewhere you will really struggle to reverse that. The reservation will show your DL number, even if DL takes the miles back. So if any other airline checks they see that apparently you credited to DL.

I don't know what this 'certificate' will be, please do check back in if you are successful, but I am afraid you won't be. Sorry for the bad news.

This is why it is best to get a new BP with the number you want to credit to on it if you ever ask numbers to be changed.