Dominance. Control. Wicked. Manipulation.
Every time hifumi asked doppo what he would have done if the roles were reversed, it reconfirmed his decision in switching the blood tests. Doppo said if he was omega he would stay away, wouldn't take medicine, wouldn't take off, wouldn't do this or that. It just cemented in hifumis mind that he made the right choice. He switched it so hed have an excuse to stay together without making doppo uncomfortable. In a way hifumi was making doppo act like an omega while thinking he was an alpha, since hifumi is very good at being manipulative. In Hifumi's mind he knew how doppo would react, confirmed it, and made himself feel even more in the right each time doppo did or answered something that proved himself correct. Switching it was his own way of showing love and keeping them together at any cost. Honestly very twisted, but gotta love it.
but there was no point in switching, doppo loves him and would do anything for him, so what was the reason?