What are the notable differences between Paranoia XP (2005) and Paranoia Troubleshooters (2009)?
I have not played either, however there is a thread on RPG.net that discusses many of the same things: The RPG.NET thread on the topic
I've quoted some relevant parts below:
"Troubleshooters is the 25th Anniversary edition of the Mongoose 2004 PARANOIA rulebook (formerly known as XP). There are minor tweaks to the rules, but it is basically the same game. Likewise, the companion Internal Security rulebook uses the same system, though there is extensive new material there for BLUE-Clearance IntSec missions. Neither is mechanically complex, and both place unique emphasis on the Gamemaster's supremacy. Every rule in both books exists as a non-mandatory advisory to the GM's incontrovertible authority.
Troubleshooters, in particular, adapts the XP GM Screen's "mission blender," a collection of tables that lets you generate an entire mission randomly with the roll of a mere five dozen or so d20s. Troubleshooters also includes Ken Rolston's classic introductory PARANOIA mission "Robot Imana-665-C" as well as a fine new mission by Gareth Hanrahan, "The Quantum Traitor."
"If you have Paranoia XP, you can play Paranoia: Troubleshooters - the mechanics and content are 90% identical. The only substantial difference is that Troubleshooters returns to the old Treason Point system, with the more complex "Treason Damage" mechanics reserved for Straight games.
As for rules for mission structure, Troubleshooters is pretty much the same as XP, apart from some expanded GM advice and some fun tables for randomly mission generation (which sometimes even produce sensible results). "
Other comments made in the thread indicate that the official development blog has many insights into the changes.
In June 2009, Mongoose Publishing announced that they would be retiring the books in the XP line to clear the way for the 25th Anniversary Edition line - revealing a new edition of the rulebook as well as two new rulebooks, one casting the players as higher-clearance Internal Security investigators and one as Ultraviolet High Programmers. They stated that the XP material would "maintain a 90% compatibility rating with the new Paranoia books".
Each of the three books is an entirely self-contained and playable game: Paranoia: Troubleshooters, Paranoia: Internal Security, and Paranoia: High Programmers. The Troubleshooters volume presents a slimmed-down version of the XP rules, the most notable difference being the removal of the Service Firms and the advanced economy of the XP edition, with the focus firmly on the game's traditional premise of casting the player characters as Red-clearance Troubleshooters.
The most noteworthy rules difference is the return of treason points. Paranoia XP had an adaptation of the combat system for making accusations of treason (the General Hostility Formula.) Troubleshooters removes this, returning to the "assignment of treason points, with summary execution upon debriefing" model from previous editions.