If I were thinking of switching anybody it would be
Minsc for Keldorn (mainly out of personal dislike for Minsc) and Jaheira for Anomen (the same reason but with greater intensity and the added fact Anomen is a more useful character).
The continued popularity of Jaheira continues to amaze me. I can only assume people take her along out of some misty-eyed reverence for the original Baldur's Gate, in which to be perfectly blunt she was at best a middling character in terms of usefulness. In BG2 she is not only outclassed as a Fighter by many characters but also as a druid, by Cernd the dead-beat dad.
Lonelypilgrim is right about the effectiveness of Anomen, sadly many people are but off by his rather arrogant personality, yet strangely these same people are usually rather forgiving of the abrasive Jaheira?
As others have said Keldorn is useful, if not slightly smug in his moral superiority, but he does have some quite interesting interjections with Anomen.
By contrast Valygar is quite a reticent character, and while he does interact more readily with characters like Mazzy and Nalia, i honestly don't remember him interacting that much with many of your existing crew. You could consider a line-up like this ...
Sauron
Minsc - your comments seem to suggest you like him
Keldorn
Aerie - whiny but incredibly powerful so I can forgive her
Anomen
Yoshimo (replaced by Imoen *shudders at the thought*)
OK I've probably upset enough people, so I'll shut up now.
