When editing your SmartObject, and changing the property, after you click Finish, the following error is shown:
This may happen when you have a SmartObject property of type Text, and you want to change it to Date\Time, while data exists within the SmartObject.
This is expected behavior, and not an issue with the K2 Product.
The reason is, K2 cannot convert existing data into different data types. You will experience the same issue when you try to change existing String data to Int within SQL Management Studio.