Sforno on Deuteronomy 13:1/ספורנו על דברים י״ג:א׳

1לא תוסף עליו, for you might add something which is despicable in the eyes of the Lord. If, perchance, you would add new ways of the serving the Lord, you have no assurance that such innovation is acceptable to G’d, for instance the burning of one’s children in G’d’s honour.

2ולא תגרע ממנו, even if, in your judgment, the rationale for observing the commandment in question no longer applies, is no longer relevant. An example would be Solomon’s reasoning that the restrictions placed by the Torah on the number of wives a king may have did not apply to him as none of these wives could possibly incite him to turn away from G’d. (compare Sanhedrin 21).