Is there a way to find out if you have a previous ADE ID?

I have had a Kobo eReader for many years.  Recently I tried using ADE for a library book and could not remember my ADE ID as it had been many years.  I created a new one but now books I purchased with the previous ID are no longer transferring from ADE to my device.  How can I find out the previous account?  Can I merge the two?  If I can't find the previous account can I change the account that the books are associated with?