Aim : do the migration for N cubicweb instances hosted on a server to another with no downtime.
Prerequisites : have an explicit definition of the database host (not default or localhost). In our case, the database is hosted on another host. You are not migrating your pyro server. You are not using multisource (more documentation on that soon).
Possible enhancements : use right from the start a pound server behind your apache, that way you can add backends and smoothily migrate by shuting down backends that pound will take into account.