וַיַּעֲמִידֵ֣ם לָעַ֣ד לְעוֹלָ֑ם חָק־נָ֝תַ֗ן וְלֹ֣א יַעֲבֽוֹר׃
He hath also established them for ever and ever; He hath made a decree which shall not be transgressed.
Rashi on Psalms
He issued a decree to them, that this one should serve by day and that one should serve by night. which will not change That decree.