It's impossible to alter value of a PK

[LeonMelis]

Using the following schema as example:

Note that for the PK autoIncrement is not set.

When trying to change a primary key:

$item = ItemQuery::create()->findPK(1);
$item->setPrimaryKey(2);
$item->save();

Will result in a query:
UPDATE item SET id = 2 WHERE id = 2;

Assuming that ID 2 not exists for this example, the query would affect 0 rows.

in model->doUpdate() the buildPkeyCriteria() method is used to create the selectCriteria, it builds a selection criteria based on $this->id as the value, but $this->id is already set to the new value by setPrimaryKey(), resulting in a WHERE clause containing the new ID, where the old ID would be needed.