Thank you for the details Nathan. I will contact Volusion directly. But to summarize for any other users happening upon this thread:

The answer to your question is that no, we are not planing to change all of our URLs at once. This will be an ongoing need, perhaps maybe 10 URLs per month on a continuing basis. The scenario is that in the past, when we made an engineering change to our products, we did not generally issue a new product code in Volusion. But for tracking, warranty, and UL listing purposes, going forward, we need to begin issuing new product codes when engineering changes occur. So, when we make a form/fit/function change to a product 12345-A, we will need to create a new product 12345-B.

The solution of canonical relationship links is not applicable here, because we are not just talking about SEO. Any visitor who arrives on 12345-A need to be taken to 12345-B. The canonical will help the search engines avoid duplicate indexing, but it is just as important for us to manage the visits of real users (who may arrive from links other than those in search engine results). So, we need a redirect. Volusion does provide a method for this (http://support.volusion.com/article/using-redirects-products-volusion), but this method issues a 302 redirect which is not desirable for SEO purposes.

And Volusion also provides a method for administering 301 redirects, but only when the requested page does not exist.

So, in the reply from Nathan above, he is recommending to physically delete the old product from the database so that the original page does not exist, and set up redirects using the Volusion application, after the deletion has been done.

There are some details to be considered when taking this approach, so we are going to contact Volusion directly to work through those. But I wanted to summarize here to help others who may have a similar need. Nathan, if I've incorrectly summarized, please add-on. Thanks.