Receptional_Andy - 8:08 pm on Mar 25, 2009 (gmt 0)

I've found Google to be extremely forgiving of downtime, particular if the pages are either entirely unavailable, or return a 50x HTTP status (e.g. 503 Service Unavailable).

My expectation is that the worst you may see is a minor blip in rankings, with everything returning to normal. To be honest, I've had pages retuning errors for much longer than a day which were not affected in rankings.

I think Google's thinking is that taking any severe action for a (likely temporarily) unavailable page would be counter-productive, and I would agree with that.

Of course, you'll have missed some crawling which may have a slight impact, and if there are any differences between the old and new, you may have other problems.