What exactly changed with Dialga and Palkia's origin formes?

Question

Aside from the obvious design, what exactly changed in terms of stats, abilities, blah blah blah?

Answer 1

Dialga will have a 20 point decrease in it's Attack while getting a 20 point increase in it's Special Defense. In addition, it's signature move Roar of Time will be slightly altered. More specifically, it's power increases while it's accuracy decreases. It also increases the chance of going fewer times in battle, so use it with caution.

Palkia will have a similar 20 point loss in it's Attack while getting a 20 point gain in it's Speed. Palkia's signature move, Spatial Rend, also changes slightly. The power and accuracy of the move are both decreased, but in exchange, its chance of landing a critical hit increases from a +1 modifier to a +2 modifier, meaning that you will be landing critical hits even more than normal. (The boosted critical hit rate increases even more when using Spatial Rend in Strong Style, becoming +2 for normal Palkia and +3 for Origin Palkia)

Answer 2

You can look at the Pokedex to see, but in summary:

Dialga: 20 less Attack, 20 more Special Defense. Taller and heavier than the normal form.

Palkia: 20 less Attack, 20 more Speed. Taller and heavier than the normal form.

That's it. They have the same type, moves, and abilities in the game data (even though abilities are not used in PLA).