Marcos, This answer might be best put over on the postGIS listserv, but I will suggest a couple of strategies based on some differing assumptions and certainties.
One certainty - it seems that you absolutely have to have some form of PostGIS installed in order to perform the postgreSQL upgrade using pg_upgrade. One variable - whether or not you will have to do the "hard upgrade" required by the postGIS version when doing the standard PostgreSQL upgrade. In other words -- is the postgreSQL upgrade essentially doing the same as the "hard upgrade"? If YES - then install postgis 1.3.x on your 9.2.4 then use pg_upgrade, then do the pgis hard upgrade If NO - install postgis 2.x on your 9.2.4 then use pg_upgrade
This is a very relevant case study, one that I myself will be going through very very shortly here so I am interested to follow your progress and assist in whatever way I might.