I downloaded the file from your signature area MrPhil.
Starting from the part where you said we had to "do a fresh install of Frozen, and then migrate your database (and other data, such as images) over to the new system." Even that is new to me. Is there anywhere that explains this more step to step? The person that installed OS Commerce on the Lunar Pages host died 2 years ago, leaving us with no teaching or information on how to do any of this.
I have 3 sites to be concerned with under public_html of our host. I am completely unsure ow to install Frozen under one of those existing domains, get it set up, then migrate old to new so that any of it will continue functioning. The owner DOES NOT want to lose ANY SEO or LINKS from the old sites.