  We show that the graph transformation problem of turning a simple graph into
an Eulerian one by a minimum number of single edge switches is NP-hard.
Further, we show that any simple Eulerian graph can be transformed into any
other such graph by a sequence of 2-switches (i.e., exchange of two edge
pairs), such that every intermediate graph is also Eulerian. However, finding
the shortest such sequence also turns out to be an NP-hard problem.
