I don't really think it matters. It's just a voice, it's not like the person is stepping in for a real person role. Re casting for voice overs happens all the time. I don't think it'll be a big deal once you're relaxed and watching the movie without trying to notice a difference.

Found this interesting comment that was posted yesterday over at NarniaFans.com:

Reepicheep’s new voice seemed more character appropriate in keeping with the book and film, and is still very likable, considering his pseudo-Grecian heroic quest – something King Caspian needs to ponder, based on the little dialogue included.

Spy Report Link: http://www.narniafans.com/archives/8466

EDIT:
I saw after posting this that Lady_Liln had posted this yesterday in GMD topic Re: Biola Conference Footage *plot spoilers*. But it definitely belongs here as well.

Very cool information you got there GlimGlum.

I knew something like this was going to happen and that people really should not fret over a voice change.

I will have to find out how well MR. Nighy does Reepicheep before I make my true opinion on this matter.....I hope he does well!