From the Huffington Post:

George Zimmerman’s self-defense claim could be hurt by his own witnesses, who have changed their accounts since they were interviewed early on in the Trayvon Martin case.
The Orlando Sentinel reported that four witnesses’ statements regarding the Feb. 26 shooting changed significantly when they were interviewed a second time in March. The statements are included in the collection of evidence officially released by the State Attorney’s Office last week.

The Post speculates that this will be bad for Zimmerman, and the changes in the witnesses’ stories do seem to be heading in that direction. (For example, one first said she wasn’t sure who was on top during the fight, then later said she was sure it was Zimmerman.) However, the prosecution has the burden of proof here, and witness testimony will be very important if the physical evidence doesn’t become more definitive than it is now; the fact that so many witnesses have changed their minds about what they saw could help establish reasonable doubt.