Is is possible to resize but to maintain the old edge positions?
Well, sort of. If you resize a node by specifying the desired width or height in yEd's properties view or properties dialog, the edge positions are not changed.
Applying the bpmn auto-layout does (to me) incomprehensible things to this graph.
Unfortunately, the BPMN layout algorithm cannot handle message flows that connect to pool nodes.
Does applying the bpmn layout change the endpoint of the edges to the center of the pool instead of its border?