On our SharePoint server, we had created several sites based on
site/list templates. Later on, we decided to use customized site/list
definitions (using the onet.xml, schema.xml, ...) for all new sites and
lists.

Now we would like to move the old sites (created with templates) to new
sites (using definitions). Is there any way to do this? We basically
need to copy all the data from the old sites to the new sites, and
ensure the new sites were created from the definitions.