You should be able to rename a Pokemon any number of times in the game.
Go to the namerater in Lavender Town or rename it right after you catch it.
In Eterna city where the second (grass type) gym is, go into the apartment block and on one of the floors there is an old man who will rename your Pokemon. He will not rename Pokemon that you got in a trade though.
In slateport city there is a name rater who can rename your Pokemon remember Pokemon traded to you can't be renamed.
Just go to Exploration Record on the main menu and select Rename Team.
In Eterna City, (the town with the second gym) beside the Pokemon shop, there is a building, go in and talk to the old looking man, then he will let you rename your Pokemon.
You can rename a pokemon by going to the Name rater
you go two the name rater in the town where the Pokemon tower is
Go to the Name Rater in one of the cities.
Yes you can, you go to Castelia city and go to the farthest left alley. Go into the first building to the left. Inside will be a man that looks like an artist. Talk to him, he'll say that he is the name rater. Though you can not rename traded pokemon.
Go into your computer to Bill's PC, and then to "Move Pokemon". When you click on the box, one of the options should be to rename it.
you go to your Pokemon then you click the Pokemon you want type its new name into the left hand corner and select 'change'
You can rename pokemon when you first obtain them or ask the Name Rater to change it. The Name Rater is in Lavendar Town he's in a house below the Pokemon Center. Bring the pokemon you want to rename to the Name Rater then he will let you rename it. Keep in mind pokemon obtained through trade cannot be renamed.