Updating a site without losing SERP

[IndianIsand]

Hello,
I developed a website a few years ago and it is doing well in its niche. Sadly there's flash involved and its horrible at mobile responsiveness. I was wanting to migrate to Genesis probably Eleven40. My problem is the extensions of the pages end with .htm. This may sound like a minor detail but remember this site is ranking really well in its intended area and its actually doing what it was designed to do which is getting new customers for these guys. I have some valid concerns regarding losing faith of the indexing bots once I migrate to something new. such as a .php externsion. I'm a little under the gun as this week is THE week when I start the redesign of the site.
I'm concerned about inserting a bunch of 301 redirects as I have a suspicion they don't taste good to the search indexing bots.
Is it possible to leave the pages such as the boat hauling page with a .htm extension and then go on to the php bases Genesis framework?
Or is there any other advice I can get here to help me with my dilemma ?
Thanks and Be Well,
William