I have a PR3 website that's doing just okay. It's been on the existing server for about a year. If I want to change hostings, would that affect PR? Or SER?
It shouldn't - if you keep the same URL. Anyhow, if you have good reason for changing host - change host even if that is going to cost you some traffic at first. Having a reliable host that gives you enough room (bandwidth wise, for instance) is more important in the long run.
as long as the downtime isn't for an absurd amount of time you should be just fine. if you want to be careful you can arrange it so there will be no downtime at all.
yes, i can try and make sure that the downtime is not much. the url won't change. this host actually lost ALL my 6 months data (articles, posts, listings, etc)! and i dropped out of MSN - the only search engine where i was ranking well. i don't want this to happen again.
in most cases a loss of traffic is due to a poorly coordinated move in hosts that involves downtime, even when search engine traffic wasn't affected in any real way at all-- they were still sending traffic at you, but the site was unreachable