The version number of on head is always one number higher than the version number of the release branch (as mentioned, needed for apm management).
The recommended action is certainly to upgrade pg8.4 to 9.*. this is typically just a pg-dump + pg-restore (since pg 9.0, there is pg_upgrade). Pg documentation [1] contains an example for using pg_upgrade from 8.4.
[1] http://www.postgresql.org/docs/9.2/static/pgupgrade.html
