Will it murder your rankings since all your pages will be changing? Like.. say i am using X CMS... and then Y CMS comes along and i want to switch...
It most probably will. Depends on what the back end of the CMS's looks like. If they have a same way of displaying stuffs, then will not make a diff. If not, it definitely will. However, you can overcome this. It's an over head solution. Why would you want to change your CMS. If it's some functionality requirement, why don't you try and incorporate it in the present one? Just Curious. ~G
It depends upon whether your permanent link structure changes, and whether you remove content or not. I have switched a few times, and never had any trouble. It just took some planning and setup work ahead of time to make sure everything went smoothly.