Hi, I've already made almost same upgrade (7.4->8.1) under Debian linux. On Debian the 8.1 database structure will be installed into /var/lib/postgresql/ by default and on 7.4 it's /var/lib/postgres/. So if you remove the packages and you (or your package manager) do not remove the databases physically you only need to reinstall the old package (without owewrite the old sturcture, (move the directory to somewhere if you want to be sure)) and the fallback will fast and safe.
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