How much would it mess with Shiro’s head if he realized at some point, when Kazui is at that age, he looks very similar to how Ichigo used to look and, by extension, how he looks?
omg it would probably hit him like a truck all at once. It would probably be a totally normal day, he’s sitting there talking with Kazui and, without any thought at all, accidentally calls him Ichigo and he’d just stop in the middle of whatever he was saying, realization dawning. He’d probably accidentally call Kazui by his father’s name at least a few times after that. And his sense of time is so bad, that surely wouldn’t help matters. He’d feel really bad about it honestly, because he hated it when people referred to him as Ichigo or relating to Ichigo in anyway, which he would then realize hadn’t happened in a long time. The whole thing would be so strange and surreal for him. Honestly, he’d probably have to find a quiet evening to seek Ichigo out and tell him about it because he just wouldn’t know what to do or think.