I don't think there is a way for one workflow to call another. Workflows are mostly run by page code. For example, if you search all the files in your web site for "WebAppSettings.WorkflowIdPaymentChanged" I think you'll find that the Payment Changed workflow is run by BVAdmin\Orders\Default.aspx.vb, BVAdmin\Orders\ReceivePayments.aspx.vb, and IPNHandler.aspx.vb.

The way I figured this out was to start at a likely place (BV Admin > Orders > Payments) to get the page name (BVAdmin/Orders/ReceivePayments.aspx). Then I read the code to see which workflows it ran and found that it did indeed run the Payment Changed workflow (using BusinessRules.Workflow.RunByBvin(context, WebAppSettings.WorkflowIdPaymentChanged). So then I searched the rest of the site for the unique string (WorkflowIdPaymentChanged).

Since the BVC5 API can also supports running a workflow using it's name, I also searched the site for "Payment Changed" but did not find any workflow runs. Since WebAppSettings.WorkflowIdPaymentChanged is just a constant, I also search for the value (db14f186-4005-4726-9a37-d67c2dc284ec), but only found the constant definition. So I'm convinced the workflow is only run by the three pages listed above.