I agree that it is ridiculous that there is no easy way to change synced libraries!
I basically had the same thing happen to me as first in thread, in trying to reset synced library I instead deleted most of my desktop library. Forced to go back to last backup only costs me a couple hours of work, but unbelievable to me that it is supposed to work this way.
So I can only sync to a single library? Have to essentially reset the endnote online system to change libraries? This seriously limits the functionality of online library (I would say online libraries, but that is pural and apparently not allowed).