if Gojo is switched, he knows he's from the future/past and he needs to explain it, because as much as I believe adult reader wouldn't be too surprised by seeing teenage Satoru because she saw him already, that teen reader might get a little... confused, seeing that hunk of a blindfolded man instead of her classmate/boyfriend.
Above that, I see it happening without reader's knowledge - Gojo is usually alone on missions, so there would be a different version of him coming home (would teenage Gojo even know where to go in the future? to school, probably).
On the other hand, if it's the reader that is switched, I assume adult Satoru would know immediately - his six eyes would detect it and he would drop whatever he was doing to find his little teenage girl (he'd tease the hell out of her, I'm damn sure of it). And then, would teenage Gojo even be capable of acting properly when instead of his pretty girlfriend that he just barely started dating there's an adult woman telling him that she is that pretty girl, just decade older.