Code

var doc = new XmlDocument();
doc.Load(file_path);

var nodeElements = doc.SelectNodes(@"/Diagram/Nodes/Node");
if (nodeElements != null)
{
	foreach (XmlElement element in nodeElements)
		if (element.Attributes["Class"].Value == "std:ShapeNode")
			element.Attributes["Class"].Value = "std:SvgNode";
}

diagram.LoadFromXml(doc);

foreach (var node in diagram.Nodes)
{
	var svgNode = node as SvgNode;
	if (svgNode != null & svgNode.Content == null)
		svgNode.Content = ...;
}