I got this note on the Amazon Developers discussion list

The easiest way to tell if you're using the older ECS 3.0 is to look in your code for calls to the Perl module. If there are any references to "modes" and you see modes all in lowercase like "books" or "toys" then the code is using the older version of ECS which will stop working soon. The latest version of ECS ... err ... A2S (no confusion there, huh?) replaces "modes" with "Search Indexes" and the search indexes are in proper case like "Books" or "Toys."

I inspected the Net::Amazon code. As far as I can tell, it uses ECS 4 on calls to Amazon, even though they use lower case in their own calling parameters. They use the following URL http://webservices.amazon.com/onca/xml?Service=AWSECommerceService . Its name implies it uses AWS, which means that I do not need to redo the calls to Net::Amazon on my site. I hope I am correct, otherwise I will have a catastrophic failure when ECS3 is turned off in March 2008.