It might be worth your time listing compatibility on the overview page of each template. I just purchased a template and use 1.5 in Non Legacy mode. if I had known that the template wouldn't work with 1.5 I would have either waited to make a purchase, or would have found another template - and saved myself the cash at the same time.
I hope that you decide to give the template updates to former purchasers.
I should have checked the forums to see about compatibility before I made my purchase - it didn't occur to me - as I didn't see anything listed on the site in the faq or elsewhere to lead me to believe there would be a problem.
kinda bummed I made the purchase for 2 URLs...
