301 redirect and mod_rewrite with page name changes

There's one confusion going in my mind
I'am workin on 1 website which is ranking very good on google and getting some good amount of traffic and revenue.
Now, the project is on .cfm and we are updating the whole backend as well as site structure into .php and than will mod_rewrite to .html pages
But the issue is my pages www(dot)mydomain.com/pagename.cfm will be conversted to www(dot)mydomain.com/pagename.html and www(dot)mydomain.com/pagename.cfm is ranking on google and i dont want to lose traffic.
So what is the solution for the same ?
Should i 301 redirect the old ranking pages to .html new pages ?? bcoz the old .cfm pages have PR4,5,6

Please help me with this issue

Thanx