Not sure why, but this is not documented by Ariba. "changes" is a vector of type LogEntry. LogEntry unfortunately is not documented in the JavaDocs. But you can look at the methods within your development tool (Eclipse or something like it).
In Java code, I iterated through the list of changes and ran logic based on a fieldName
change.
fieldName.equals("Amount")
You can get the old value and compare to new, etc.