According to the chain, the guarantee is part of a larger effort of "Re-Colonelization" that also includes retraining every manager and cook in the 25-minute process for prepping and cooking the chain's Original Recipe fried chicken. The changes come as KFC looks to draw back "everyone who grew up with the familiar taste of KFC and has turned away from [it] in recent years."
The guarantee is available at participating KFC locations with a limit of one replacement per customer per day. The fine print also states that the replacement offer is only valid within 24 hours of the original purchase at the location of purchase (to cover up any loopholes I suppose).
KFC's guarantee sounds similar to Papa John's recent quality guarantee but actually goes further as it covers the subjective taste of the product whereas Papa John's only applies to whether they made it properly (and not whether you liked it).
Here's the video from KFC detailing the changes and guarantee:
Image via KFC.