Well, I think that saying "...to naru", the change is more radical. There is a big change, but using "...ni naru" the change is kind of normal. I think that saying "sanagi kara chou to naru" would be better, because sanagi´s body becomes very different from the original. Saying "watashi wa sensei ni naru" I think this is normal and it is not a big change. I think it would be better to say "ashita wa watashi no tama ga terebi to naru". This change is not natural, so "...to naru" would be better. I don't know if I am right. I cannot explain why 3+5=8 "ni / to could be used". I would use "ni", because it is normal 3 + 5 = 8. I don't know if you are understanding me.

please explain more detail or example. Anyway 3+5=8 san tasu go ha hachi to naru. 3+5=8 san tasu go ha hachi ni naru. both ok However sanagi(a pupa) kara chou(a butterfly) to naru.←△ why△sometime this phrase is used poem or something.... sanagi(a pupa) kara chou(a butterfly) ni naru.←○ ashita ha ame to naru←△ ashita ha ame to naru deshou←○ ashita ha ame ni naru←○