That means doing an exchange. First check if your pass can be exchanged.

how can i tell if my passes can be exchanged? i can’t find it anywhere.

If the Eurail account works like the Interrail account, then you can find your invoice under payment information in the order overview. On the invoice there should be information about the conditions of your pass. 

I had the invoice in the confirmation mail (PDF attachment) and there it says indeed whether or not an exchange or refund is possible.

@Adena Astrowsky 

I suggest that you buy the 1st class Eurostar reservations before you make the exchange of your pass. Otherwise there is a risk that you get a 1st class pass and still can't travel as the pass holder seats have sold out. 

So can you keep your 1st class eurostar reservation made with a different pass?

Sure, there's no link with your pass, only with your name.

@Ingrid Widdows yes as long as you travel with a pass valid in the correct class