That's why the trend seems to be (and I'll follow for now, as an easy way to migrate) to grab free Google Apps on your domain.
You still need to backup your mails (run an imap/pop client somewhere) in the case that Google locks you out. But in that case mail to you is targeted at _your_ domain, you just drop the DNS entries for GApps and set up your mails _then_.
(More clever: Set it up before, but don't list it as MX. That way as soon as DNS propagates you're able to receive mails again)
You still need to backup your mails (run an imap/pop client somewhere) in the case that Google locks you out. But in that case mail to you is targeted at _your_ domain, you just drop the DNS entries for GApps and set up your mails _then_.
(More clever: Set it up before, but don't list it as MX. That way as soon as DNS propagates you're able to receive mails again)