pg_upgrade (formerly called pg_migrator) allows data stored
in PostgreSQL data files to be migrated to a later PostgreSQL
major version without the data dump/reload typically required
for major version upgrades, e.g. from 8.4.7 to the current
major release of PostgreSQL. It is not required for minor
version upgrades, e.g. from 9.0.1 to 9.0.4.

pg_upgrade works because, though new features are regularly
added to PostgreSQL major releases, the internal data storage
format rarely changes. pg_upgrade does its best to make sure the
old and new clusters are binary-compatible, e.g. by checking for
compatible compile-time settings, including 32/64-bit binaries.
It is important that any external modules are also binary
compatible, though this cannot be checked by pg_upgrade.