In 1984, in addition to having changed physically, how else has Winston changed since his arrest?
This depends where you are in the book. By the end of Book 3, Winston has completely changed in 3 ways.
First, Winston denies the need for relationship with Julia. Before he had been tortured, Winston had somewhat of a relationship with Julia and it contained a type of commitment. Maybe it was as much commitment as possible in that society. After an opportunity to see her, he had no desire to maintain or re-ignite the relationship he had with her.
Next, Winston does not have a will of his own anymore. He used to think and wonder and consider. Now, he cares not of his own desires, but in a lackluster fashion, is willing to do what the party requires in the way that they require it to be done. We also note that his interest, and those things that intrigue us about his character as on-lookers have vanished.
Finally, Winston now loves, instead of despises Big Brother. He had been broken and he learned to have affection for the idea of this governmental leader which likely didn't even exist.