If you are experiencing issues connecting Liferay Sync to the upgraded Liferay Portal, you'll need to reinstall Liferay Sync in order to reset the Sync settings.
In Windows, Sync can be uninstalled through Control Panel > Add/Remove Programs > Liferay Sync. With a Linux/Mac, this can be achieved through the terminal through the rm -rf ~/.liferay-sync.
The newest version of Liferay Sync can be downloaded here. Simply uninstall the old version of Sync, then install the new version and reconfigure the Sync settings when prompted.