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

[andrea_r]

http://codex.wordpress.org/Permalinks

Genesis does not control the page extensions - Wordpress does. You can have .htm at the end of the links, just like we have on the Studiopress blog.

[IndianIsand]

Originally Posted by andrea_r

http://codex.wordpress.org/Permalinks

Genesis does not control the page extensions - Wordpress does. You can have .htm at the end of the links, just like we have on the Studiopress blog.

Wow thanks for the speedy response Andrea.
Are you saying to set it under Permalink settings and then set it in Custom Structure. Is this how it was done on the blog? I'm sorry if this isn't a Genesis question but I'm under the gun here to get this ball rolling this week. I actually have a meeting Thursday.
I posted a question in the Wordpress forum but I don't think its as vibrant or knowledgeable as here.
Thanks
PS here's a link to the home page and then one to the Boat hauling page
http://longislnadmarine.us
http://longislandmarine.us/boathauling.htm

[andrea_r]

Are you saying to set it under Permalink settings and then set it in Custom Structure. Is this how it was done on the blog?

Yes.

I posted a question in the Wordpress forum but I don't think its as vibrant or knowledgeable as here.

Yes, it is. :) I'm a mod there as well. It is staffed by volunteers, not employees. They are far more knowledgeable there.