g1smd - 5:46 pm on Apr 25, 2012 (gmt 0)
I'd estimate this thread saved you more than a week of research, coding and bug fixing. :)
Long term I would aim for extensionless URLs. Staying with .aspx URLs for now is the least disruptive as far as Google goes.
Google deals with URLs. It matters not one bit what the filename of the file on the actual server hard drive is called. If the URLs are the same, they will have no idea that something inside the server changed.