XHTML is XML, so.... rename -v -n 's/\.html$/.xml/i' * (remove the -n for this to actually do something).

But, seriously, as you said, RSS is XML too. So you're going to have to interrogate your clients better. Ask them what they want the output to look like or what spec it should conform to, ask them to show you an example, etc. - whatever questions it takes for them to give you enough information to do your task.