It would be incredibly difficult for a runner at second base, given the fact that he’s not going to be there very many times, to decipher a catcher’s sign pattern. Major league catchers generally always run a pattern of signs, with one sign being the “Flag” that indicates the next sign is actually the real pitch to be called. So if 1-Fastball 2-Curveball 3-Slider 4-Changeup 5-Flag, a catcher will show 2-2-5-3-4-1 when he wants a slider; the flag says “the next sign is the real one, everything else is bullshit.” Coach signals (touch the bill, rub the arm, touch the bill, wipe the chest, etc.) usually work essentially the same way.
The idea that the catcher and pitcher change their signs is probably fanciful on the part of the announcer in terms of pitch selection - in all likelihood what they ARE talking about is the placement of the catcher. Without a runner on second the catched can move inside or outside without the batter knowing; however, a runner on second can easily signal to the batter of the catcher is setting up way outside or some such thing. So the catcher and pitcher will get together on where the catcher will set up and where the pitch might actually be, to cross up the runner.
That said, it’s possible some players WILL change sign patterns because there seems to be a growing paranoia about sign stealing. You’ll also notice pitchers and catchers holding their gloves over their mouths so you can’t read their lips - despite the fact that there is no reason to believe that anyone in the major leagues today, or for that matter in the entire history of the major leagues that I am aware of, has EVER successfully divined the other team’s plans by reading lips. It’s ridiculous, and yet it’s a practice that’s spreading like wildfire.