Ana or Anna as a nickname ? See below.

My daughter's name is Anastasia, although she was born Anna and we chaged it to Anastasia a year later because we wanted a more formal and longer name.

But still cal her Anna or Ana..... She is 3 and the teachers asked me how to spell it.

Should it be spelled Anna (the name se had when she was born) or Ana (a more natural abreviation of Anastasia)?