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So in answer to your question, I'd say about now would be a good time.
Here's what WebGuerrilla says about it:
[webmasterworld.com...]
I've got sites that always get visits later than everyone else it seems, and they're done by now, including the one that got lost last month. So I'd agree with Olias.
I moved a section of that site with a 301 and grabbed that in just before the update started, so it should be cool for the coming update, that's how I timed that one.
I just moved a few sites, as long as you leave the site up at the old host until Google updates it's DNS cache then it goes smoothly.
I was worried about moving in case googlebot came and subsequently kicked the sites out of the index, but it didn't happen.
I could see Googlebot in the logs from the old host and then when I started seeing her in the new logs (new host) then I removed the pages from the old host.
Because Google caches the DNS and takes normally around 14 days to update the DNS then even if your site has a bit of downtime due to the DNS propagating google will *still* be going to the old host as if nothing has happened, and then, by the time Google updates its DNS cache your site is back up on the new server.
Hope this makes sense ;)
Dazz
(too slow :) )