Webwork - 12:13 pm on Sep 7, 2010 (gmt 0) [edited by: Webwork at 2:51 pm (utc) on Sep 7, 2010]
The advice I've most often read is to (slowly) 301 permanent redirect old site/pages to new site/pages, keeping an eye on the new site's performance. (I'm not 100% certain that "slow redirecting" is universal advice.)
Since it's a charity you might have some success asking inbound linkers to point their links to the new site/pages.
Finally, I'm not sure if there's (an effective) way throught G's webmaster tools to "tell G" about the move.
Any other ideas/suggestions?
[edited by: Webwork at 2:51 pm (utc) on Sep 7, 2010]