how do you modify the value of the primary key of a record?

So whenever i try a MODIFY i get the error " "Table.Field" 'NEW_VALUE' does not exist " seems like it tries to find the record with the new primary key value which obviously doesn't exist yet in the table... so how am i supposed to change the value of the primary key field then? Do i have to delete the old row and insert a new one? seems weird to me.