When you have a piece with the time signature 12/8 that switches to 6/8, is it important to be aware of this change? Or do you just play the notes? I understand that when 3/4 switches to 6/8 this does have impact, but does it really matter when for example 12/8 switches to 6/8 and vice versa? The feeling shouldn't be that different right?
The feel should change slightly but in a pretty subtle way.
In 12/8 time, there should be emphasis on the first beat and a smaller emphasis on the third beat (which is the 7th eighth note (quaver) of each measure).
In 6/8 time, there should be an emphasis on the first beat of every measure, which means that every other beat will have an emphasis that is a bit stronger than it would have in 12/8 time.
12/8: O . . + . . o . . + . . 6/8: O . . + . . O . . + . .
One way to hear how 12/8 feels different from 6/8 is to compare "Dazed and Confused" by Led Zeppelin (in 12/8) and "Vitamin R (Leading Us Along)" by Chevelle (6/8). The tempo of the Led Zeppelin is slower, but you get more of a compound 4/4 feel overall, while the Chevelle is clearly a compound march (2/4). This is audible even though they have similar basic beats of kick-snare-kick-snare. Listening for end-of-measure fills and start-of-measure crash cymbal hits helps demonstrate the difference.
If a composer has taken the trouble to change from 12/8 to 6/8, HE must feel it makes a difference. So YOU shouldn't just ignore it. Agreed, it's not as big a difference as 3/4 to 6/8. But it's 'ONE-and-a two-and-a three-and-a four-and-a' changing to 'ONE-and-a two-and-a, ONE-and-a- two-and-a'. That's different.