IMO it depends what you mean by big and also on how much you like the existing design. I have converted my 40page site from no css to external style sheets quite succesfully, and using Frontpage 2003. Was it easier than starting from scratch? I think so, but only because I didn't want to change my design and I wanted to convert one page at a time keeping the whole site live at all times. Had I been up for a design change as well then I'd have started from scratch.