Who is permanently changed by the magical midsummer night in A Midsummer Night's Dream?

Well, the most obvious answer to your question is of
course Demetrius. It is he out of the four Athenian lovers who is changed permanently in
terms of where his affections lie. All the others, after having been changed back,
retain their original affections, because Lysander and Hermia still love each other and
Helena still loves Demetrius. It is only Demetrius that has his affections permanently
changed from loving Hermia to loving Helena. Note what Demetrius says to Theseus as an
explanation to his baffling change of affections in Act IV scene
1:

...my love
to Hermia,

Melted as the snow, seems to me
now

As the remembrance of an idle
gaud

Which in my childhood I did dote
upon;

And all the faith, the virtue of my
heart,

The object and the pleasure of mine
eye,

Is only
Helena.

Demetrius thus
dismisses his former love for Hermia as "the remembrance of an idle gaud." It is he who
has been changed permanently by Oberon's magic. However, equally, we could argue that
Titania has been changed as well, as she awakens from her "dream" to being in love with
Oberon, whereas before there was only discord in their
relationship.