You're game is glitched. Get a new one on eBay.
Press select when your pokedex is on then go to shift en then in the last section you can change it
Beat Wallace/Steven
Latios's Number ID in the national pokedex is 384. He is found in Pokemon emerald, rubr and sapphire. He is constantly moving when you change routes or go into cities and buildings. He will flee immediatly.
first get all the Pokemon on your normal Pokedex then go to the professor and you will get the nationaldex.
If you want to change one, you'll have to restart your entire game. However, if you manage to complete the ENTIRE Hoenn Pokedex, you can visit Prof. Birch and he will give you ONE of the three Johto starters. Please note that this is only in Emerald, not Ruby or Sapphire.
Basicaly the Pokemon change in there, but I don't know how to change the Pokemon.
No.
no!
you can't
it's random
First of all, congratulations on getting the national dex, now addressing your question: The significant change is the pokemon available in the Safari area. you can now catch pokemon such as Wobbufett, Houndour, and mareep in the safari zone. Smeargle also is available in the Battle Frontier. Asides from that though, not much. Good luck and have fun nevertheless :D
by getting the national pokedex.