Hi Edmund,
If you unfortunately have lost important notes after you upgrade, you have 2 options:
Get them back from a Time Machine backup:
To do so, choose Enter Time Machine from the Time Machine menu, or click Time Machine in the Dock. And use the timeline on the edge of the screen to locate a version of the Notes storage folder that just precedes your deletion. Click Restore to restore the selected file, or Control-click the file for other options. When you next launch the Notes app, your missing notes should reappear.
Get them back from the Storage file:
To recover your lost or deleted notes from storage on Mac, at first, you need to find Notes’ storage location at ~/Library/Containers/com.apple.Notes/Data/Library/Notes/. Enter the path string in the Finder’s window. And you will see the Notes folder, inside this folder, you should see a small assortment of similarly named files with names such as NotesV2.storedata.
Copy these files to a separation location, and add an .html extension to them. Then, open one of the files in a web browser, and you will see your current and the deleted or missing notes. At last, copy and save deleted notes to a separate location. And done !
Hope this helps you,
Cheers !