For a tax class change to be possible, both partners need to be "unbeschränkt einkommensteuerpflichtig", which means to be subject to an unrestricted income taxation. In practical terms, this is made possible only when the partner is also a German resident. However, an exception to this rule is possible if you and your partner fulfill the following requirements (source): • You have EU or Swiss citizenship. • Your partner has residence in another EU country (+ Switzerland). • At least 90% of your common total income is subject to German income tax. The same applies if the income not subject to German income tax does not exceed 19 964€ in a calendar year.