This page shows the 8 unique valid bipartite graphs for 5 agents and 5 pieces after quotienting by isomorphism. In the isomorphism action, a1 is fixed, while a2..a5 can be permuted and b1..b5 can be permuted.
Starting from a graph, if we remove the critical edge (red) and add the appearing edge (green), and the resulting graph is valid, then the transition is not critical.