From experience i can say that changing a composer, when you've had the same one for so long, does effect the game. Frank Kelpacki worked for Westwood Studios and wrote all the music for the Command & Conquer games. But after EA bought them out he left and took his talents elsewhere, and you hear the difference in Command & Conquer 3 and 4. He did come back and work on a couple songs for Red Alert 3, and again, you can tell.
Also Akira Yamaoka, was the composer for the Silent Hill games(and the movie), but he left and there is a lot of speculation over what the next game will sound like.
With Half-Life, I really hope they can capture the essence of the music as we have heard it. I've rated HL2 as one of my top 5 soundtracks, ever. But TF2, L4D, and Portal all have great sounds to them, so I'm hopeful.
And I've never played Mass Effect, so I don't have any comment... :/