The Original names of the 4 guys were Jesse (Carlos), Konas (Logan), Bryan (James) and Donovan (Kendall).
Kendall Schmidt was originally turned down for the part of "Kendall" and Curt Hansen was casted instead, but Logan told the producer to audition Kendall a second time. After his second audition the producer asked Carlos, James, and Logan who would they pick Kendall or Curt. The guys picked Kendall and the producer gave Curt the part of Dak Zevon in the series.
In the unaired pilot episode in which Curt Hansen was "Kendall", the guys were skaters, they lived in Wisconsin, Carlos doesn't wear a helmet, and "Katie" is 16 or older. After Kendall Schmidt got the lead they changed the script. Katie (now played by Ciara Bravo), Kendall's mom (Challen Cates) and a few other things. Also adding the Turd Song and the fighting scene with security in the audition.
There's an unaired pilot episode in which Curt Hansen was "Kendall" and they sing This Our Someday instead of Big Time Rush. It was revealed by Carlos Pena that they replaced Curt because of the lack of chemistry between them (they have to become friends) and they added Kendall Schmidt two weeks before the episode was supposed to air.
At the audition, James Diamond's number is 812 and Kendall Knight's number is 813. In later episodes with them wearing yellow and red/brown hockey jerseys, Kendall's number is 13 and James' number is 12.