Is it permissible for a scholar to change a legal ruling under threat of death, such as permitting usury, for example?

No one is permitted to alter the rulings and laws of the religion; otherwise, they would be fabricating lies against Allah and His Messenger. Allah has promised severe punishment for liars, stating: "And do not say, concerning that which your tongues falsely describe, 'This is lawful and this is unlawful,' to invent a lie against Allah. Indeed, those who invent a lie against Allah will not succeed. A brief enjoyment, and for them is a painful punishment." So, whoever declares as forbidden that which is lawful, or as lawful that which is forbidden, when these are established and necessarily known aspects of the religion, is considered a disbeliever and an apostate from Islam, unless they are under compelling coercion, where the coercer is capable of carrying out their threat, according to His Almighty saying: "Whoever disbelieves in Allah after his belief, except for one who is compelled [to do so] while his heart is at rest in faith."