I requested an email that confirmed the change but did not call I feel I have it in writing that should be enough.
BEWARE if you buy a ticket and want to refund it your balance will be 0, because they will look at your payment (the voucher will be invalid) that is what happened to a friend he lost $700 that way !!!!!
It is different if you just want to change dates!

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Viele Grüße

Oliver