what is different trace and trace trace

If there was a value for the given attribute or intensity prior in the cuelist (you see a minus - ), [Trace] tracks the new value backward to the last move instruction.

If there was no value before (no minus), [Trace][Trace] can force the change to the beginning of the show (or the next blocked cue in this direction)

If there was a value already, [Trace] and [Trace][Trace] act the same way.

Maybe a little video helps:
www.screencast.com/.../4ab2i8Yag9

Has there something changed in 2.8 or 2.9.?

I'm pretty shure, in 2.7 and prior, the command in the picture below had forced channel 2 to be at 40% from cue 22.5 to 26.
Now you have to put [Trace][Trace] to achieve this.

I always used ChanX [Update][Trace][Track] if I wanted it to be at a certain level the entire show, for example blue lights or something like that...

Has there something changed in 2.8 or 2.9.?

I'm pretty shure, in 2.7 and prior, the command in the picture below had forced channel 2 to be at 40% from cue 22.5 to 26.
Now you have to put [Trace][Trace] to achieve this.

I always used ChanX [Update][Trace][Track] if I wanted it to be at a certain level the entire show, for example blue lights or something like that...

No, it's been that way as long as Trace has been a function.  Sammy's explanation above is correct.  if a channel is inactive in the cue, and gets a new active value with an Update Trace, the console will not trace the new value back and will just dump them in the cue.  This function does not look at cue list ownership (the "-" that you reference above).

If you want to update the values back regardless of if they're applying to an active or inactive channel, you have to use [Trace][Trace].