If you change the value of a record's primary key when using SYNCHRONIZE, in fact the old record will still exist but a new record will be created with the new value as its primary key. If this is not desirable, the correct solution would be to delete the old record and create a new record (with appropriate data of course).

The replication feature (SYNCHRONIZE is just two replications) relies on the uniqueness of the primary key value. A new primary key value means a new record in all cases.