Grids are absolute positioned. That means you can't make sure they are always centered. Web design does take some planning and if wanted your site centered you should have used a different method to lay it out.
Does every page have to be redone? That depends. Did you plan ahead and use a template to create your site pages? If yes, then all you have to do is change the template and GoLive will automatically redesign all the pages. If you didn't use a template, then yes, every page will have to be redone (Make a template, then copy/paste your content into it).