It might also be a good idea to add "Conflicts: gaim < [some version]" so that you can't also install an older version of gaim, which would probably fail because of file conflicts at install-time.
Okay, pidgin 2.0.0 (final) is now in G:C (2.0.0-39.1 for distro versions 10.1 and upwards to Factory) and it *should*, all being well, automagically upgrade gaim to itself.