I have run my site for 13 years now. I have been on page 1 of Google for 9 of those years. I have been on Page 1 placement 1 out of 64 million search results for 8 of those years.
My site is static, handwritten content. I have avoided WordPress, Joomla, Drupal, and all the others when it comes to key pages:
1) My home page 2) My static unchanging content pages (the main core of the site)
These pages *might* consist of 30 to 40 pages maximum.
I then tacked on PostNuke (many years ago) for a news section. I then tacked on vBulletin for a forum. I then tacked on Magento for a store.
So my site is static, with plug-ins (basically). This has served me well, and I often tell myself that I ranked so well for so many years because every bit of my code on the core pages, was 100% unique and hand written from scratch.
Now its 2013, and my users want a global site login. My site needs to work on mobile platforms and ipads. My site needs to use HTML5 because I'm told thats the wave of the future. My users want to post images and participate on the forums and purchase products with one login.
So I am terrified ... but im thinking about a CMS system. I dont want to drop vBulletin and I'm not dropping Magento. I need something better than PostNuke for the news center. But do I really need to go 100% CMS? Probably not. Probably can still have a static home page and static core content while carrying a logon session right?
What about using the CMS functions of vBulletin? I know of people doing that with great success and it would keep your changes to a minimum.
I would say this is not something to fiddle with trivially: "I have been on page 1 of Google for 9 of those years" Kudos!
Personally, I would, however, say this is a myth: "I ranked so well for so many years because every bit of my code on the core pages, was 100% unique and hand written from scratch. "
So balancing those two considerations, I would want to be very careful about rolling things out and try to keep the static pages as close to the original as possible as you integrate them into a CMS if you do at all. You might find that with a little bit of scripting you can pull some dynamic elements onto those pages and keep thems eparate from the CMS.
This seems critical decisions for you and many competitors are behind and challenges are there to adopt new technology. Adopting new technology is good sign but you have good traffic/ranks from the existing. My suggestion is to integrate without hampering the ranking values because in future you need to change as par the visitors demand, otherwise there will be loss.