honestly, i don't think it will be the way we expect it to be, if it happens at all... depends more on which Sukuna it will be. an old reincarnation? a child? someone we already know in modulo? or a meeting after Yuuji's possible death? these are all different scenarios in my head... i'm sure Yuuji wouldn't be scared, not with his experience and the power he has now. maybe he would be surprised. there would definitely be the question "what are you doing here" hah... if Sukuna was just reincarnated to start over, i can imagine Yuuji would take it with a light smile... and try to keep his distance. because it's easier that way.
but if Sukuna was reincarnated because of/for him, because there's still a connection between their souls... i want to let myself dream and imagine how much that would squeeze Yuuji's heart, the same one Sukuna once gave him. because he was never meant to be this lonely, and that's what Sukuna has to fix