Heres the update page, but with only one update behaviour. And currently the trigger is something different.
I am setting a session variable earlier on in the checkout process called 'checkoutUpdateBilling', which will be set to true or false.
Let me know if I'm barking up the wrong tree.
I realise now, I can just make two update pages and use session to manipulate link on prior page.
But if there is an easy way with trigger, that would be a useful tool.