A minority, who for one reason or another don't know about the change, will be gobsmacked when they get Gerry's face onscreen instead of Pat's.
The very first shot, establishing Gerry Ryan as the one-night presenter of the Late, Late Show, will change the way people watch the show.
Instead of simply reacting to one item after another, the conversation in sitting rooms and pubs will be a series of comparisons.
"He did that interview very well."
"Yeah, but he fumbled the bit about the competition."
"Tell-ya, he's giving Pat Kenny a run for his money."
"Oh, come on, he's OK as a substitute, but ... "
Viewers who don't like Pat never call him Pat. They call him Kenny or Pat Kenny. Those viewers will be rooting for Gerry Ryan as if he was auditioning for the gig.
It will be crucially different from the two occasions Gay Byrne's Late, Late Show was handed over to someone else. Once, when Gay became unexpectedly and seriously ill, a frequent panelist on the show, Ted Bonner, stepped in. Lovely man. Great guest. Not a presenter.
The other time was towards the end of a show about feminism, when Gay ushered the young Marian Finucane into his seat to handle the rest of the show. Lovely woman. Great radio presence. Good presenter.
But that's not the real difference. The real difference is the shift in the way programmes such as the Late, Late Show are produced, these days. Twenty years ago, they were live chat shows. You never quite knew what was going to happen, and neither did the presenter. Today, the show is much more regulated.
Gay's researchers, for example, would meet guests in advance.
"This guy gets emotional about his father," they'd tell their presenter. "Push him about his Da."
Researchers working for Pat and Ryan Tubridy don't do that. "If you ask him this question," they say, handing over the cue cards. "He'll give you this answer." It's safer.
It's also much less exciting. It doesn't generate the same televisual great moments or emotional car crashes.
But it suits both presenters. Pat is at his best with current affairs interviews, Ryan in situations where he can produce witty one-liners.
Neither man is comfortable with emotional meltdown, although each dutifully pokes guests with the unspoken plea: Go on. Cry for me.
Gerry Ryan, on the other hand, is never happier than when he's sitting in a puddle of tears or indiscretions or rudeness or chaotic conflict.
On radio, that is. On TV, he has tended to do long interviews with famous people, pre-recorded. Tomorrow night, the pressures of constant instructions in his ear-piece, "Wind this up, flag the competition and get ready to stand up and go to the woman in the red jacket in the front of the audience", will undoubtedly rob him of some of the freedoms that suit him on radio.
He comes to tomorrow night's programme on a wave of personal publicity resulting from the publication of his autobiography. According to newspaper reports, he protested to the editor of the RTE Guide this week about the hostile review that publication gave the book. But that's unlikely to halt his gallop on the Late Late.
If he plays it right, Gerry Ryan will not even attempt the clinical orderliness of Pat's programme management.
He'll make a dog's breakfast of some of the links. And it won't matter. Because he'll concentrate on getting to the emotional hot-spots and creating the conflictual chaos in which he thrives. Good luck to him.