Hello, I found a solution for this problem.

I changed the onchange function that happens in the line of a One2many so it sets a flag that it is changed.

Then I added a onchange function on the "order_line"-field in the order model. This onchange method is triggered as well after a line changed and it is triggered the last in line after a change in the records.

In the onchange-orderline function I check the states of the flags, reset them and preform the necessary action using a function call on the sale object. (don't know if it is necessary to do it with a function call, but I saw that in sale.order.line onchange fiscal postition)

That does the job. The lines are changed on-screen!

class SaleOrderLine(models.Model):
    _inherit="sale.order.line"

    categ_changed = fields.Boolean("technical field, do not use or show to user", default = False)

    @api.onchange('categ_id')
    def _onchange_categ_id(self)
        for line in self:
            line.categ_changed = True
        return True

    def change_categs(self, changed_lines_lst)
        for line in self
            if self.id in changed_lines_lst:
                self.categ_changed = False
                #do necessary update actions within this line
        return

class SaleOrder(models.Model):
    _inherit="sale.order"

    @api.onchange('order_line')
    def _onchange_order_line(self)
        #we need to get this from te context because object might not be the same as screen values.
        ctx_lines = self.env.context.get('order_line')
        changed_lines_lst = []
        if ctx_lines:
            for ctx_line in ctx_lines:
                if ctx_line[2]:
                    if ctx_line[2].get("categ_changed")
                        changed_lines_lst.append(ctx_line[1])

           for order in self:
                self.order_line.change_categs(changed_lines_lst)

adding context to view:
<xpath expr="//field[@name='order_line']" position="attributes">
    <attribute name="context">{"order_line": order_line}</attribute>
</xpath>

Good luck :-)