Hello Arthur LEHAVRE ,

Use this code you no need to override the is_company field.

class ResPartner(models.Model):
    _inherit = 'res.partner'

    @api.onchange('company_type')
    def onchange_company_type(self):
        rec = super().onchange_company_type()
        self.is_company = False
        return rec

Hope this helps!

Thanks & Regards,

Name : Yahoo Baba

Email:  yahoobaba077@gmail.com 

Thank you ! Indeed it works great ! And is there way to set it up in my model ?

i think you can use user defaults 

activate debug mode

open the contact page 

press on debug icon

click on user defaults