OK, sorry, I misread your first post.
You can deregister your old ID from ADE, using ctrl-shift-D (cmd-shift-D on a Mac).
Restart ADE (maybe not needed), and then reregister with your new ID.
However, be aware that once your have done this you will not be able to read any DRM books set up with the old ID.
If you also have eReader devices you wish to change,
deregister them while plugged into ADE using ctrl-shift-E (cmd-shift-E on a Mac).
Restart ADE (maybe not needed), and then reregister the device with your new ID which is already registered on the PC/Mac..