The subject of the fragment tells about present unreal condition. It is not true now and may not be fulfilled and he or she just imagines about what the situation would be if the condition were fulfilled. There is a clear indication of that in this sentence: It would be even better if you were here.
The structure of present unreal conditional is If ... Simple Past ..., ... would + verb ...
So would just can't go inside the if clause. And so the right answer must be changed. Except of polite requests where would is possible in if-clauses.
But this is a letter someone writes to another person. And telling about imaginary situation now he or she just politely asks another person to join. And there is a clear indication of that I wish I had tried harder to persuade you.
So the right answer is would change.
And also had changed can't be the right answer. Because in this case it would be past unreal conditional which tells about what happened in the past so you can't change it (it happened in the past) and you just imagine about what the situation would have been if the condition had been fulfilled in the past. But this isn't true because the subject tells about present condition which still can be fulfilled though unlikely and imagines what the present situation would be if someone were here.