Belgium amended its constitution in 1991 to make succession to the throne gender neutral, applying the new rule to the descendants of Albert II. It was a modest piece of drafting with a long reach: it decided, a decade before she was born, that the eldest child of the future King Philippe would be heir regardless of sex.
That child is Princess Elisabeth, Duchess of Brabant, born in October 2001 and heir apparent since her father acceded to the throne in July 2013 following the abdication of Albert II. If the constitutional order holds she will be Belgium's first queen regnant, in a monarchy that has had seven kings since 1831.
The timing of the amendment matters more than the text. Several European monarchies changed their succession rules across the same period, and most did so with a reigning generation already in place, which meant the change either skipped over people or was written to avoid disturbing them. Belgium legislated before the question was live, which is the cleanest way to do it and the least likely to produce a grievance.

