phranque - 10:37 am on Mar 17, 2009 (gmt 0) this is a good rundown of the procedure for changing servers.
as far as changing the server company, that is a matter of specifying the new DNS that points to the ip address of the new server.
that all happens with the domain registrar, so you will need the username and password to access the account at your registrar.
change the password when you get it.
Matt Cutts: Moving to a new web host:
this is a good rundown of the procedure for changing servers.