Igraph's internal data representation changes sometimes between versions. This means that it is not possible to use igraph objects that were created (and possibly saved to a file) with an older igraph version.
The input graph.
The graph in the current format.
graph_version queries the current data format,
or the data format of a possibly older igraph graph.
upgrade_graph can convert an older data format
to the current one.
graph_version to check the current data format version or the version of a graph.