Yes, you can upgrade, but an upgrade means a new build of your site because the platforms are very different. It's similar to comparing Windows with Macintosh computers.
If you wish to upgrade to the latest version 5.0, you will need to submit a ticket and let us know that you want to move to the latest version. The designs and platforms are different. Support is provided for customers in exporting data from the old system and importing it into the new system, and we can give you a quote for us migrating your data, if you need our service. We can create a staging site for you so that you can build your site in that staging environment while keeping your current site live.
The costs are the same as what you have now. There are no additional costs to use our latest Ozcart.
We plan on remaining with this version of Ozcart from now on and updating it with new features and bug fixes. That will give you more stability.
A demo of the new cart is available at: https://ozcart.com/try-demo
Please discuss your needs with us, if you are interested.