InvoiceChangeGateway Hook

Hi,

I'm using a hook to change the currency of the invoice when changing the payment method.

Hook: InvoiceChangeGateway

I can get the variables successfully but I couldn't find a way to change the currency of the invoice or the currency of the client.

I want the steps to be in this way:

1- Client opens the invoice.
2- Changes the gateway.
3- The currency in the invoice will be changed to what I coded.

I can't get the hook at anyway to change the currency, do you have any suggestions?

Thanks,