See, you've hit on exactly the right question: It's not just that the actor changed. The entire demeanor and nature of the character transformed in a most radical way. Gene had Jeff overmatched this episode from minute one. We remember the Jeff of yore as a confident, even cocky, guy who is inherently threatening, despite his dimwits, thanks to the information he holds over Gene's head. Then, here, Jeff is instantly cowed, enticed and enlisted as a partner in crime in just seconds of screen time. So, the question becomes: Why did they change Jeff's character, not just the actor portraying him? I think the answer is fairly obvious, but you gotta follow the thread a bit. First, Jeff in S4/S5 is a threat, and threats are storytelling gold. Drama is tension and conflict. That's "Screenwriting 101"-type stuff. So, you don't drain all the tension out of a story, after going out of your way to create it in the first place, unless you've gotta really good reason to do so. For screenwriters of this caliber, there's simply no way this is an oversight. They did it knowingly. Now: What did they gain from doing it? How did neutering Jeff from the moment he appeared on screen this season change the story? Well, the show is about Jimmy, and this put Jimmy back in the driver's seat. It brought Jimmy back to the "clever grifter who can get the best of anyone" role we'd reveled in back before the cartel and grim death. And this shift in plot and power dynamics accompanied an equally seismic shift in the show's tone - a tonal U-turn back to S1/S2, worthy of Kim on D-Day. The writers could've renewed the conflict with Jeff, stoked it, brought it to a head, but instead they chose to go light tone and low tension. The greatest source of conflict in the episode came not from Jeff, but the security guard! We abruptly returned to the lighthearted, mischievous Jimmy of old, but one who ultimately rejected reverting to full Saul. And I think that's the pay off. If we retroactively reconstruct this, I think it's clear the writers wanted Jimmy to begin the journey to break free from Gene and take the first steps towards returning to "himself" - i.e. the early seasons Jimmy, but pre-Saul. If he'd spent the episode breaking out every ruthless trick in the book to cut down Jeff, then his rejecting the Saul clothing at the end indicates he returned to those tactics when threatened, but doesn't want to embrace them permanently. By returning first to quintessential Jimmy, then having him reject Saul-type clothes, it makes an entirely different statement - "No, I'm not Gene. But I'm not Saul, either. I'm stickin' with goodhearted Slippin' Jimmy. That's who I am." To get there, though, you can have Gene locked in combat with Jeff the whole episode. You have to erase basically the whole essence of the story they'd setup for Jeff, short of simply pretending the thing never happened.