The edges in a graph are always presented in the same order (sort first by source node, secondarily be target node). You can maintain an index into the previous numbering of the edges by adding a new property that tracks this:
>> g = graph;
>> g = addedge(g, 5, 6, 1);
>> g = addedge(g, 1, 2, 2);
>> g.Edges
ans =
2×2 table
EndNodes Weight
________ ______
1 2 2
5 6 1
