tedster - 11:06 pm on Feb 12, 2013 (gmt 0)
You can be more at ease about Google than just a few years ago. The number of ranking factors that are not dependent on your mark-up is way up. This doesn't mean that you can't mess things up - it's still certainly possible. But it is not as common as it once was. Just develop your new site in a test environment be fastidious about quality control before launch.
One big place to mess up is definitely URL problems, especially introducing new but unintended versions of URLs that still resolve. This kind of thing can act like a slow poison to a site, rather than an immediate and obvious issue - and that makes it nasty.
Whether you can easily preserve existing URLs of not depends on many factors, but you are best off to look for ways not to introduce URL changes - especially at the same time that you introduce new mark-up. Of course, some people just bite the bullet and change everything at once, thinking "I'll just get the pain over with and then deal with whatever happens."
That's a brave approach but I've seen businesses end up happy with the final results. At least it let's you move the focus back to content and off technical issues a bit faster. But if you go this way, you should project an income drop and have a financial "war chest" available, just in case you need to bump up advertising after a loss in organic traffic.