This is if you see Java classes in red and get this error, "Cannot resolve symbol".
If you're importing projects into IntelliJ and none of the above solutions worked for you then give this a try. This is what worked for me when everything else failed.
Go to to your project folder and rename/delete the .idea folder which has the idea settings for your project. This would have been created from your old IntelliJ version. Once you have renamed/deleted the .idea folder, import your project into IntelliJ. You should not see any errors for your Java classes now. Hope this helped.