> You need to set up streaming replication (yes, it works across Postgres versions) slave
Unfortunately no. Streaming Rep doesn't work across versions. Streaming rep requires that the wal version number/magic, the catalog version number and several other identifying factors are the same between both ends.
SR/HS work by replaying the wal from the primary on the standby. Unfortunately its format changed in at least each of the last 5 releases. Even 9.3 - with 9.2 being in beta atm - already has significant changes to the format.
> upgrade it (in-place upgrades are fast), let it catch up with the master
And unfortunately, while pg_upgrade upgrades are way much faster than the traditional pg_dump/pg_restore, they aren't that fast because they require the planner statistics to be rebuilt. A whole database analyze can take a bit on a larger database.
Don't get me wrong: I *really* like postgres. I really like SR/HS. I even like that pg_upgrade exists although the way it works isn't that elegant from my POV.
Unfortunately that doesn't make those features appear :(.