How to mangage change of mailserver without mail loss

[pipex]

What happens to mails sent to a domain e.g. someone at adomain.com when the
Internic DNS entry for the domain and the MX record is changed to other IP
addresses.

As far as I know DNS entries have a life between 1-7 days. So does it mean
mail would go to the old IP address until the DNS change is propagated to
all nameservers in the world?

Is is possible to make changes to the MX record before the IP switch so that
mail starts going to the new destination.

Comments please.