can anyone possibly help?

Set up the new cart on a "test" or "dev" sub-domain, and add a small number of sample categories and sample products to it.

Use .htpasswd to password protect this site so that no one but you can look at it.

Evaluate all of the URL formats that the new cart uses for categories, products, and so on. Install any necessary URL rewriting packages first.

If you can get the category and product numbers and names to be the same as the old site then the job is a lot easier.

Once done, set up the .htaccess file such that requests for old URLs are redirected to the new URLs.

Thoroughly test all of these. Pay attention to both www and non-www requests and make sure that any request results in a single-step redirect, not a multi-step redirection chain.

Once testing is complete, set up the entire shopping cart with all of the categories and products.

If you can't get the category and products names and numbers on the new site to be the same as those that were on the old site, an entirely different approach is called for.

In that case, you will need to make a special "redirect script" that can look up old URL requests and find the new URL to match it. You then rewrite all "old" requests to this script and the script sends the redirect to the new URL.

This usually entails setting up a database of old vs. new data. For a small database, you can use phpMyAdmin to edit it. For larger projects you might want to build you own special "admin" front end.

In this latter case, there are very few lines of code to go in the .htaccess file, all of the heavy lifting is done in the "special" PHP script.

Once you have all the testing complete, and the new site fully loaded with products, you then move all the scripts and database data to the live domain.

Before doing that, you move the live site to a "backup" sub-domain so that you can reinstate it in minutes if something goes wrong with the new site in the first few days.

You'll also need to import all your customer and order data from old to new site, and it's a good idea to block new orders and new signups for a short while before you start moving things about.

You do not want someone placing an order on the old site moments before you switch to the new site and the data being "lost" in the change.

thank you so much for the great advice!
In your first example (method)....

"Once done, set up the .htaccess file such that requests for old URLs are redirected to the new URLs."
For this to work I would assume that the database id's for products and cats would need to be the same?

I cant visualise a way to link the 2 together otherwise?