Finally, I have completely overridden the field 'report_type' and its methods. It works well.

In my custom module :

class account_account_type(osv.osv):
 
    _inherit = "account.account.type"

    def _get_financial_report_ref(self, cr, uid, context=None):
        obj_data = self.pool.get('ir.model.data')
        obj_financial_report = self.pool.get('account.financial.report')
        financial_report_ref = {}
        for key, financial_report in [
                    ('asset','account_financial_report_assets0'),
                    ('liability','account_financial_report_liability0'),
                    ('income','account_financial_report_income0'),
                    ('expense','account_financial_report_expense0'),
                    ('personnal expense', 'account_financial_report_personal_expense0'),
                ]:
            try:
                financial_report_ref[key] = obj_financial_report.browse(cr, uid,
                    obj_data.get_object_reference(cr, uid, 'account', financial_report)[1],
                    context=context)
            except ValueError:
                pass
        return financial_report_ref
    def _get_current_report_type(self, cr, uid, ids, name, arg, context=None):
        res = {}
        financial_report_ref = self._get_financial_report_ref(cr, uid, context=context)
        for record in self.browse(cr, uid, ids, context=context):
            res[record.id] = 'none'
            for key, financial_report in financial_report_ref.items():
                list_ids = [x.id for x in financial_report.account_type_ids]
                if record.id in list_ids:
                    res[record.id] = key
        return res
    def _save_report_type(self, cr, uid, account_type_id, field_name, field_value, arg, context=None):
        field_value = field_value or 'none'
        obj_financial_report = self.pool.get('account.financial.report')
        #unlink if it exists somewhere in the financial reports related to BS or PL
        financial_report_ref = self._get_financial_report_ref(cr, uid, context=context)
        for key, financial_report in financial_report_ref.items():
            list_ids = [x.id for x in financial_report.account_type_ids]
            if account_type_id in list_ids:
                obj_financial_report.write(cr, uid, [financial_report.id], {'account_type_ids': [(3, account_type_id)]})
        #write it in the good place
        if field_value != 'none':
            return obj_financial_report.write(cr, uid, [financial_report_ref[field_value].id], {'account_type_ids': [(4, account_type_id)]})    
    _columns = {
        'report_type': fields.function(_get_current_report_type, fnct_inv=_save_report_type, type='selection', string='P&L / BS Category', store=True,
            selection= [('none','/'),
                        ('income', _('Profit & Loss (Income account)')),
                        ('expense', _('Profit & Loss (Expense account)')),
                        ('personnal expense', _('Profit & Loss (Personnal expense account)')),
                        ('asset', _('Balance Sheet (Asset account)')),
                        ('liability', _('Balance Sheet (Liability account)'))], help="This field is used to generate legal reports: profit and loss, balance sheet.", required=True),
    }