I have been messing around with throwing up some of the older 1.0 version of the themes as Demo sites and to my chagrin the .xml files don't work, the images are no longer available etc... but then I got to thinking, the older forum discussions are sort of left mixed into the discussions for the new versions until nobody discusses the old anymore.
I would like to suggest that when a theme is completely revamped, for example let's look at Streamline 1.x to Streamline 2.x. Upon Streamline 2.0's release you would create a sub-forum under the Streamline forum called "Streamline 1.x" and then move all topics, tutorials etc... to that sub forum. In the case of something like Church which was replaced by a new theme all together, you could do the same, move the Church forum beneath its successor for organizational purpose.
This way, whenever anyone needs to discuss that particular theme, or needs to find something pertaining specifically to that version of Streamline it would make their posting and searching more simplified and generate less noise in the results. I am sure many people still have a lot of sites running on the older themes and do not plan on upgrading those sites. Not only that a theme like Streamline 1.0 still has a great look yet it is completely different than Streamline 2.0.