Thanks for your reply. But I never use iTunes Match (it is always off), so this cannot be the problem.
Since my original post, I have found that iTunes 11.0.2 WILL allow me to change the media type of podcasts. BUT for about half the podcasts I regularly subscribe to, iTunes will not reliably allow me to change the title and other ID3 tags when the file is new (within a few days of the file's download). I can change the name (for example if I want to archive the podcast with the date in its name) but at random intervals thereafter all the tags I have modified will revert back to the way they were before my edits. This happens even if I have moved the file so it is no longer in the original folder where iTunes downloaded it.
Frustrating, as I have to edit tags as often as five times before the changes stick. This never happened with previous versions of iTunes.
Don't get me started on the mess that searching has become now. It was perfect in iTunes 10, where search results would fill a window and included results for all fields from all types of files. Now you get the drop down menu which disappears the moment you change focus, and includes only a fraction of the results it once did.
iTunes 11 = massive FAIL.