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.