Well, it may be a little late but if you read my latest article in my ASP.Net section of my site you'll see why a certain reluctance to make this change came about.
It has always been a laborious task to make such a change, especially if you want to apply some sort of master page strategy to the HTML pages on the site.
Being a lazy person at heart, rather than do the job with repetitive and boring hands-on editing I have created a little utility program that can convert all the .htm pages on a site to ASPX while applying a master page if required.
I ran this on my own site and it seems to be OK, at least for my purposes. I still need to tidy some pages but the bulk of the work was done automatically. Just right for a lazy old duffer like me.