The Pokédex order is stored beginning at 0x41024 in Red/Blue or 0x410B1 in Yellow (both bank 10). The first entry is 0x70 and the second entry is 0x73. This means that Rhydon; the Pokémon with index number 0x01 is 112 (0x70) in the Pokédex and Kangaskhan; the Pokémon with index number 0x02 is 115 (0x73) in the Pokédex.
I've created Game Genie codes that change Pidgey and Rattata for this per request.
Replace XX with the new Pokédex number you want, converted into hexadecimal.
Rattata as No. 238 in Red:
Rattata as No. 238 in Yellow: