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An alternative is to make a similar implementation as Cash on delivery where AdminOrdersControllerCore is setting $defaults_order_state according to PS_OS_COD_VALIDATION set in ps_configuration table:

$defaults_order_state = array(
	'cheque' => (int)Configuration::get('PS_OS_CHEQUE'),
	'bankwire' => (int)Configuration::get('PS_OS_BANKWIRE'),
	'cashondelivery' => Configuration::get('PS_OS_COD_VALIDATION') ? (int)Configuration::get('PS_OS_COD_VALIDATION') : (int)Configuration::get('PS_OS_PREPARATION'),
	'other' => (int)Configuration::get('PS_OS_PAYMENT')
);

This is the way it is implemented in PS1.6.1.17.

In my case i updated PS_OS_COD_VALIDATION from 14 (pending payment at delivery) to 3 (preparing order) directly in the database.