If the upgrade was 4.19, yes you can upgrade. If you do, you should:
go to the Model Download link and download the new version of BOINC,
suspend and exit boinc,
backup the boinc folder (just in case),
run the new 4.19 setup executable allowing it to overwrite the previous version.
At that point it should restart from the place it left off. While most people have not had any problems with the upgrade, a few have so that is why the suggestion to backup your BOINC folder.