Re
"reassign all edges connecting to the event to the action node"
If there are edges connected to the event node, converting the event node into a label will automatically remove those edges from your diagram. In order to prevent that from happening, you need to disconnect these edges from the event node and connect them to the action node before converting the event node into a label.
Re
"adjust the reassigned edges' source and target ports so it looks like those edges connect to the label"
After reassigning edges from the event node to the action node and converting the event node into a label, the reassigned edges may not end at the new event label. To fake the impression that these edges still connect to the event, you may need to manually adjust the coordinates of their end points. The end points of an edge are called source port and target port and these terms are also used for the settings to adjust end points in the properties view.
Re
"to fix those edges' connection points for automatic layout"
yEd's layout algorithms usually change ports to match their edge routing paradigma. E.g. "Layout" -> "Organic" changes ports to the center of the corresponding nodes while "Layout" -> "Hierarchic" distributes ports over the boundaries of the corresponding nodes. However, for edges that end at event labels, you usually do not want the layout algorithm to change the ports. The port constraints tool is a way to tell a layout algorithm that certain ports should not be changed.
