Hi,

Hope you are working with v8. I don't think, you need to override the onchange function.

If your tax_amount depends only on product uom, you can define the tax_amount as a compute field which depends on product_uom. onchange is default in compute fields. If tax_amount depends on more fields, you can include that also. For eg:

class sale_order_line(models.Model):
    _inherit = 'sale.order.line'
    
    tax_amount = fields.Float(compute='_get_tax_amount', string='Tax Amount')
    
    @api.one
    @api.depends('product_uom', 'product_id')
    def _get_tax_amount(self):
        
        //Your code here that calculates the tax value
        self.tax_amount = tax value