The reeds are tuned exactly the same, seeing as they are the same reeds in both instruments. The difference is often in the all-wood construction in many of the melodeons, and size of the box. Smaller because instead of a double set of the same reed (C) that you have in a pa (for pulling the bellows out and pushing them in) in the diatonic you have two reeds (hence the name) in the same space. C for bellows in and D for bellows out.
The diatonic however is not just limited to this action if you have more than a one row box. On my three row I have the choice of playing a D in the same direction as the C. I play the C on the middle row (key of C) pushing in, and continue pushing in and play the D below it on the outside row (key of G). They say that as you get older, more challenges to the brain are helpful in keeping mentally alert. Well, I've certainly got my plate full learning the diatonic!
The beauty of the diatonic is to be able to play notes in the same direction producing a smooth legato, or to punch the note quickly by suddenly changing direction, giving a lively rythym to dances. One of the reasons diatonics are so beloved by trad folk dancers in many countries all over the world.
In changing from a pa to a three row diatonic, it's not just about learning which buttons produce which note, but also whether to pull or push for that note depending on the effect desired or the amount of air left in the bellows. The air button is also an integral part of playing when you run out of air at either end. You have to learn how to breathe the box at just the right time so as not to interupt the melody. Much like a singers who have to choose when to take a breath. Skilled players like Shand could play certain tunes without seeming to hardly move the bellows.
Playing a diatonic certainly has more challenges for someone used to playing a pa, and then switching to a diatonic, because there are more choices available when playing a tune. It can be a bit bewildering sometimes!
An interesting journey for those not faint of heart. For me, one of the main advantages is the weight, 9lbs. Which is a pretty good inducement to play more often. I will at some point get a diatonic with Stradella bass as well.
A bit long winded and off thread, but I wanted to address the question of differences of sound between the pa and diatonic. To recap, they are the same reeds tuned the same way. Depending of course on the choice of wet or dry.
Nathen