This user on YouTube, who goes by the name of retromanstv, confirmed that he remembered the day nicktoons 2004 switched to nicktoons network 2005. His explanation is:

"this was actually the last 2004 bumper before it switched and then a countdown appeared then a launch promo for nicktoons network appeared then the first show came on then they started showing promos for new shows like skyland and new bumpers and such it was amazing dude plus they updated the im kicking myself right now I saw it all but its probably lost forever I even saw the day nicktoons first aired commericals back in 05"

The last 2004 bumper can be seen here:

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