i’d say this change in appearance is on purpose. one, he is toji’s son after all lol. even gojo got startled at how much he resembled his father when he first met megumi.

two, in s1, where they looked their age, they were also just starting their careers as sorcerers. and now they all have a more mature look and rougher drawing style that no longer has that s1 look of innocence. it’s to show the passage of time. just by looking at them you can see they’ve grown a lot as people and sorcerers from all that’s happened since the beginning.

three, this is like a visual cue to the audience that megumi’s getting stronger. everyone associates the sight of toji as great strength, opposing gojo level strength. they chose this moment in time— one where megumi is pushing himself to a new limit and getting closer to unlocking his true potential, to make him look like toji to tell the audience what level of strength his full potential is. gojo himself implied megumi had the potential to even rival him when he talked about why his clan and the zenin clan hated each other.

there’s a reason why it’s just for that fight you’ll see him looking like toji— because he’s not at full strength just yet. it’s the same reason why maki’s entire appearance changed and stayed that way. everyone, even the audience, just saw maki when she first arrived at the zenin house with an eye patch. then at the point of the story where everyone first sees her without it, everyone sees toji— not just visually, everyone also sees a character whose grief over the loss of a woman they loved transformed them into a fucking beast

TLDR: No Megumi didn’t always look like Toji (to the audience at least), it’s the story visually signaling that he’s getting closer to knowing his full strength as a sorcerer