The idea is to avoid 'onchange' methods and replace them by 'compute' methods.

Instead of

    fiscal_position = fields.Many2one('account.fiscal.position', string='Fiscal Position',
        readonly=True, states={'draft': [('readonly', False)]})


    @api.onchange('partner_id')
    def onchange_partner_fiscal_position(self):
        self.fiscal_position = self.partner_id.property_account_position


we define

    fiscal_position = fields.Many2one('account.fiscal.position', string='Fiscal Position',
        compute='_compute_fiscal_position', store=True,
        readonly=True, states={'draft': [('readonly', False)]})


    @api.one
    @api.depends('partner_id.property_account_position')
    def _compute_fiscal_position(self):
        self.fiscal_position = self.partner_id.property_account_position


So the onchange is defined as a computed field that can be modified by the user.

Behavior

    an onchange triggered on 'partner_id' automatically invalidates 'fiscal_position' and recomputes it
    write({'partner_id': pid}) automatically invalidates 'fiscal_position' and recomputes it
    write({'partner_id': pid, 'fiscal_position': fp}) does not recompute 'fiscal_position'
    create({'partner_id': pid}) automatically computes 'fiscal_position'
    create({'partner_id': pid, 'fiscal_position': fp}) does not recompute 'fiscal_position'


The behavior covers:

    onchange methods: potentially all onchange methods can be expressed as compute methods
    create() and write() behave as if they had executed the onchange methods


Technically speaking

    target: master
    modify create() and write() to compute fields not provided but with a compute method
    modify the field setup:
        compute=... does not imply store=False
        compute=... does not imply readonly=True
    adapt documentation