Hi friends!
Using the procedure that I posted earlier (also link below), changes the age of the zoi and retains the Zoi’s existing relationships.
So @swiftspeedmode , you can use the procedure to make Donghoon younger, while retaining his relationship with your zoi.
There is just one thing to note. If a senior is changed to a child, or young adult, then for sometime, the zoi will continue to display mannerisms of the previous age (senior in this case) such as walking slowly with a bent back. So after changing the age, immediately make a new save, exit to lobby, and load the new save. Then the behavior as per the new age is correctly seen in the zoi.
I have added this info in the procedure below.