You should be able to run 1.7.2 and 2.0 on the same machine (even at the same time if you want).
However, installing 2.0 over 1.7.2 does do some curious things.
You often end up with 2.0 in a semi-authorized state.
First, I suggest you make sure ADE2.0 is really authorized correctly ~ whatever it says about authorization.
Try completely removing any authorization using ctrl-shift-D to ADE (cmd-shift-D if on Mac).
Restart ADE, and then reauthorize with your Adobe ID.
Verify that the eReader is still authorized with your correct ID as well.
See how it goes and post back, and we can take it from there.