It is not entirely clear what you are talking about.
If you're talking about slony replication, it is designed to replicate data across versions. I've replicated databases from 8.4 to 9.1. You must have running the same schema (regarding the replicated objects) on both servers already. Slony replicates tables and sequences very well, it has some stuff for replicating schema changes, but I prefer doing this out-of-band.
If you're talking about in-place upgrading your db server, I'd suggest reading the fine documentation and going from 9.1 to 9.2, then to 9.3 (just to be cautious)