Bug 93400 LTK UI should provide official API for structural content in the preview page
org.eclipse.ltk.ui.refactoring/src/org/eclipse/ltk/internal/ui/refactoring/ChangeElementTreeViewer.java
private ChangeElement getElement(ChangeElement element, boolean next) {
		while(true) {
			ChangeElement parent= element.getParent();
			if (parent == null)
				return null;
			ChangeElement candidate= getSibling(parent.getChildren(), element, next);
			if (candidate != null)
				return candidate;
			element= parent;
		}
	}
	
	private ChangeElement getSibling(ChangeElement[] children, ChangeElement element, boolean next) {
		for (int i= 0; i < children.length; i++) {
			if (children[i] == element) {
				if (next)
	 				if (i < children.length - 1)
	 					return children[i + 1];
	 				else
	 					return null;
	 			else 
					if (i > 0)
	 					return children[i - 1];
	 				else
	 					return null;
			}
		}
		return null;
	}
}