A Google Webmaster Help thread reports that Google Webmaster Tools seems to be missing the feature to change the method you would like to verify you site with. Now, I personally don't remember what should or should not be on this screen, but reportedly, there should be a way to tell Google that you want to switch from the html file verification method to the meta tag verification method, or visa versa.
Here is a screen capture of what I currently see for one of my sites:
A top contributor at the group said, "There was an option there in Manage Site Verification not too long ago. Now it is not there. There must have been a page code update and a new one is pending?" So, I suspect he is right.
Hopefully the feature will come back shortly.
Forum discussion at Google Webmaster Help.