I'll try and answer your question, please let me know if you need more info/clarification:
I am not sure now: will my previously entered user data be in the database if I upgrade from 1.1 to 1.5? And also any similar issues upgrading from 1.1 or 1.5 to 2.0?
The plugin contains an upgrade routine to *up*-grade between version, this means in effect that your notes (and once you've upgraded - your settings), will still be available after the upgrade. They may be saved slightly differently under the hood, but as a user you should not notice that.
The downside of this, is of course that if the saving of notes/settings changes between version (and it has), downgrading will cause problems as old versions will not know how to handle the new note/settings format.
The new way of saving notes/settings will always overwrite the old settings, so the old format will no longer be available after an upgrade.
If I uninstall 1.5, will the uninstall automatically clean up the user data in the database?
The original version which is currently in the WP repo does not remove notes/settings form the database on uninstall.
The newer versions I'm working on, *do*, starting with version 1.5. So if you uninstall version 1.5, any previously entered notes will be deleted.
It seems to be easier and safer just to copy/paste my existing notes into a text document than messing up with the database. This way I could just uninstall 1.5, install 1.1 from scratch and copy/paste my data into 1.1 from my text file.
If you only have a few notes, that might well be the easiest way if you are not very comfortable using tools like phpmyadmin.
Hope this helps ;-)