OK, now I've tried chat, this forum and a circular round of phone support. No joy. Someone must be able to deal with this?
Adobe Customer Care on Twitter (@AdoebCare) did respond to me with me this answer:
Please follow the instructions below to De-authorize ADE : Go-> Go to folder. Go to folder dialog will appear. Type ~/Library/Application Support/Adobe/Digital Editions. Drag the activation.dat file to the trash.
So that would allow me to deauthorize the CloudLibrary account. But what happens to all those books? Are they lost to me now?
Ok well the original library helpfully contacted CloudLibrary for me but all they could do was tell me how to get rid of their authorizations. They suggested I contact Adobe (uh huh) or maybe the retailer (Kobo) who might be able to reset the authorizations.
Anyone know if this is correct?
So it turns out the retailer (Kobo) can reset the authorizations. After a lot of back and forth and a submitted list of titles, they helpfully reset them and I was able to redownload the acsm files and fulfill them under my main Adobe ID.