Pokémon FireRed and LeafGreen/New Stuff
Pokémon have natures, abilities, genders, and can hold items.
The Pokémon have their Generation II and III evolutions, but cannot evolve into them without first obtaining the National Pokédex. An example would be Golbat evolving into Crobat. Magnemite now are Electric/Steel type.
Pokémon can now breed in the daycare on Four Island, and still can go to the daycare on Route 5.
Version-exclusive Pokémon and wild Pokémon have now been changed because of new evolutions from Generations II and III.
Legendary Pokémon have also changed. Articuno and Zapdos are still found in the Seafoam Islands and Power Plant. The location of Moltres has been changed from Victory Road to Mt. Ember, which is on One Island, which can be accessed after defeating the 7th Gym on Cinnabar Island. Mewtwo is still in Cerulean Cave. You can now catch Entei, Raikou, and Suicune after defeating the Elite Four. They are found roaming around the region. You can catch Entei if your starter Pokémon was Bulbasaur, Raikou if your starter was Squirtle, and Suicune if your starter was Charmander. Lugia and Ho-Oh can be caught with the event item MysticTicket, and, after the Elite Four, their locations can be accessed by going to the harbor with the ticket. You can catch Deoxys with the event item the Aurora Ticket, which will bring you to Navel Rock from the harbor. You will catch it in Attack Form in FireRed and Defense Form in LeafGreen.
 Move Changes
Movelists are updated and include the moves added in Generations II and III. The TM list is the Generation III list, not the Generation I list.
Move tutors are now available and can teach Pokémon Generation I TMs. Three new move tutor moves have been added. Blast Burn, Frenzy Plant, and Hydro Cannon can be taught to the fully evolved Kanto starter Pokémon's final forms from the move tutor on Two Island.
 New Devices
Trainers that are found in the overworld now can be rebattled with the Vs. Seeker. Information on major characters such as gym leaders are stored in the Fame Checker.
 Other Changes
The Elite Four can now be rebattled. After completing the Sevii Islands Quest, they will have Generation II Pokémon as well as Generation I Pokémon. Team Rocket has a new base on Five Island, making them the only villainous team that does anything after the Elite Four.