You can change weather to night only during mid-innings break.
But, I find that if I change the conditions to night, play a few overs, save and reload the weather again becomes sunny and savtweak doesn't work from here on.
So you need to play all night overs continuosly after changing.
By hex-editing I got to know that byte 915768 is the one that decides the weather. It is 05 for night. I did this change in an eng-saf 50 over match after 1 over of 1st innings, i saved and reloaded many times and the weather remained as night.
The 2nd innings started, I played 2 overs saved,reloaded and it was sunny yet again.
and the byte 915768 can do nothing about it(it was still 05)
Likely, we can play Night/Day matches in Cricket 2004 but not Day/Night ones.
Gr8 bug HB.