Legal analysis: It is illegal for a bank to change the loan contract without authorization, but whether it involves a criminal offence depends on the specific situation and cannot be generalized.

Legal basis: Article 144 of the Civil Law of People's Republic of China (PRC), a civil juristic act without capacity for civil conduct is invalid; Article 146, paragraph 1: the civil legal act implemented by the actor and the counterpart with false intention (conspiracy to express false intention) is invalid; The first paragraph of Article 153 of the Civil Code of People's Republic of China (PRC) violates the mandatory provisions of laws and administrative regulations and is invalid; Article 153, paragraph 2, a civil legal act that violates public order and good customs is invalid; Article 154 of the Civil Code of People's Republic of China (PRC) * * * A civil legal act in which the actor maliciously colludes with the counterpart and damages the legitimate rights and interests of others is invalid.