tpeck:
I'm still getting nowhere - but thanks for all the pointers.
What I am now doing is fresh installing 1.5.3 and rebuilding from scratch. BUT I still want to start with importing the only database I still have - which is v1.5.1.
My question now is how do I best go about importing the DB? I can do it in phpadmin, but, of course, it isn't the correct version. OR I can use the zc_install, I suppose, except that the new host advised me to install zencart 1.5.3 on their server using Installtron or some such thing, which I did and I have a bare bones version now going.
Which is the best approach do you think?
They may have advised to use their auto-installer, but those of ZC highly discourage it. Usually, because once installed modifications to personalize the store tend to result in issues. The instructions provided above are one of a good set to install or upgrade.
To keep the previous database records you will in some sense end up with the same condition of having to upgrade the database using zc_install.
Personally, if the route to install zc 1.5.3 was an autoinstaller, I would make note of the settings in the configure.php files/copy them to a local computer, remove the auto-installed zc 1.5.3 files, download the latest zc version from http://www.zen-cart.com/getit (believe it is still 1.5.3 hotfix 1), apply any changes found at: http://www.zen-cart.com/showthread.php?213868-Known-Bugs-%28and-fixes%29-with-v1-5-3 applicable to the install package, do the install using the settings that were in the configure.php file reviewed above, this will put you at a ZC installed version of the software not a onestep click who knows what they changed installation.
Then basically move through instructions related to upgrading the database.