Here are steps: http://www.strivingprogrammers.com/MonoX/Pages/SocialNetworking/Discussion/dtopic/etDRpYMR3E6FSaMpAD8-ig/upgrade-scripts-from-nopCommerce-2-80-to-the-next-version-using-web-matrix/?dmsgId=GpFvTJwUik-XaaMpAGdutA#PostGpFvTJwUik-XaaMpAGdutA
Complete steps are mentioned here also: https://www.nopcommerce.com/boards/t/29709/database-upgrade-from-nopcommerce-19-to-nopcommerce-330-without-data-loss.aspx#120929
We are able to help you should you desire. We have completed these tasks and have hardened the process. We don't mind if the you have custom code or it is a straight upgrade.