Retcon

The order of writing of works that feature or mention Valeria Matuchek was:

"Operation Changeling" (1969);
A Midsummer Tempest (1974);
"Losers' Night" (1991);
Operation Luna (1999).

In these works, Valeria:

is three years old and abducted to the Hell universe;
is a young adult and a witch (training or qualified?) traveling between universes;
is listed among women who had spectacular lives;
turns fifteen and is the first human being on the Moon in her timeline.

Thus, Operation Luna "retconned" a spectacular event into Valeria's teens. Poul Anderson did not yet know about Valeria's lunar expedition when he wrote the earlier published works.

In A Midsummer Tempest, Chapter xi, "THE TAPROOM OF THE OLD PHOENIX," Prince Rupert and Will Fairweather enter the inter-cosmic inn, the Old Phoenix. Already present are a man and a woman who continue their conversation. She addresses him as "Holger" (p. 93) and he addresses her as "Valeria" (p. 94). Thus we know, or begin to suspect, that we have met both before although Valeria is considerably older than when last seen in "Operation Changeling."

Later: In A Midsummer Tempest, Valeria is "'...on a field trip, collecting material for a master's thesis.'" (Chapter xii, p. 103)