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Remote Buddy has to decide which version to control when multiple versions of Plex are installed. The alternative would be to launch all installed versions of Plex when you activate the Behaviour - and control them all simultaneously. I can't imagine anybody wanting to do this.
I also think it's only fair to assume that if someone installs a new version of some software, they actually want to use it - so that's what gets picked.
With regards solutions, I can see two:
1) Uninstall the newer version of Plex, which you don't (seem to) want to use. Then, after relaunching Remote Buddy, it will only find the older version - and control it as before.
2) Uninstall the new Behaviour (by deleting ~/Library/Application Support/Remote Buddy/Behaviours/Plex.rbbehaviour) that added support for Plex 0.9.5 and relaunch Remote Buddy. Since you're then back to the old version of the Behaviour without support for Plex 0.9.5, you shouldn't expect it to control Plex 0.9.5, though. Do not install any Remote Buddy updates (for both Remote Buddy itself and Behaviours) until you're ready to actually use the latest version of Plex you installed on your Mac.
- Felix Schwarz