dla takes Genitive, so "dla innych ludzi". And if you "never do that", that should usually take imperfective. So "Nigdy się nie zmieniaj".
I think that maybe there are some contexts when "Nigdy się nie zmień" would sound okay, but generally imperfective is the right way.
Thank you for pointing that out, Jellei. I should've known the change for 'dla', this slipped my mind. And that nuance of zmienić/zmieniać is quite useful.