I would recommend using a different variable to store the new currency in and then mapping that variable to the specific tags that should use the updated currency.
For Example, let's assume you want to convert the "order_subtotal" variable from PLN to EUR to pass to Facebook.
Create a Set Data Values extension like so:
Create a new data source called order_subtotal_eur
Set order_subtotal_eur To: Variable -> order_subtotal
This is going to create a copy of the order_subtotal variable that you can use to convert into EUR.
Add the Currency Converter Tag and Extension.
Set up the Currency Converter Extension as shown in the screen shot. This is using the Facebook Custom Audiences Tag as an example and assuming you only want to convert this value on the Confirmation / Thank You Page.
Then you can map "order_subtotal_eur" to the conversion value for the facebook tag and any other tag you need to send the value in EUR.
Hope this help.