Need to allow user to change primary key

I''m creating program for manipulating Access database, and one of its features is that user MUST be able to change primary key column values (with some restrictions which will be implemented). But after runnig procedure I get this error: "Additional information: Failed to enable constraints. One or more rows contain values violating non-null, unique, or foreign-key constraints." Please help as fast as you can, I''m reaching a deathlock in a few days.