Hi, if you have an Interrail/Eurail 2nd class pass, you can travel on Eurostar only on “STANDARD”, with an 1st class pass you can travel in STADARD and STANDARD PREMIER. 

Train ticket offices in Italy can’t help. 

Where did you buy the reservation?

I bought it on b-europe.com. I don’t know if they will allow to pay the fare difference. There is no 2nd class pass available to exchange. Will contact customer service tmr. 

With a 2nd class pass you can only travel in 2nd class on the Eurostar so you need to get a 2nd class reservation. 

Check b-europe for available pass holder seats.

There is a limited number of pass holder seats on the Eurostar and on popular departures and during high season those sell out weeks, and sometimes months in advance.

The best place to see the availability of passholder seats and make reservations on the Eurostar is 

https://www.b-europe.com/EN/Booking/Pass#TravelWish

Press "later trains" and eventually you will find the next available connection. 

If you have a mobile pass you need to generate a Pass Cover Number in order to make the reservation at b-europe. You do that here in the PCN generator:

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653

I already got a ticket and apparently it is for 1st class, is it possible to upgrade and pay the differences with b-europe? 

As far as I know there is no upgrade available for Eurostar.

@Al_G @Yorkie Am I correct regarding Eurostar and upgrade to 1st class with a 2nd class Eurail pass?

or if it possible to upgrade my eurail pass to 1st class, however, i have activated it but i haven’t used it yet. 

For Eurostar there is no upgrade fare, but I was thinking about something other. Do they check at Eurostar the Eurail Pass? I travelled there two times, they never checked my pass. 

The only option I can see is to ask at Eurostar, who may be able to transfer you to a second class seat since it is a genuine mistake - they may need to put you on a later train though.

I’ve been traveling in Italy and Switzerland and they only check my pass once when I have seat reservation to show, but still i’m a bit scared to risk :(( 

For your information, at Eurostar there are pre departure security, ticket and border/custome check. Like in an airport.

is there ticket inspector on train checking like other European trains or they only check the ticket at the gate before boarding?  

You can apply for a refund on your 2nd class ones as long as the email you received with your passes says you can. You can buy 1st class ones but expect to pay a 15% cancellation fee on the old ones. Deactivate your passes.

Angelo, I have had mine checked in London. The gate would not open and my pass was checked by a member of staff. It looked like all passholders were being checked on this date. The next month sailed straight through!

You should not ask for a refund, but make an exchange of passes. The cost is 15 EUR per pass. This is only possible if the conditions of the pass allow an exchange of passes. You need to buy the new passes and then send both the old and the new pass numbers to Customer Support. Read more below. 

https://www.eurail.com/en/eurail-passes/ordering-info/refund-exchange-policy

Unfortunately, i activated my pass on the 24th of December which has already passed. Although, I haven’t used any of my travel days, I think it is impossible to deactivate or refund the pass.

I’m not entirely sure if it is possible for an exchange. As the term only say it is eligible if “You've not started to use your Pass and would like to exchange it for a different one.” I am not sure if this means if I haven’t used the 4 days or if I haven’t activated the pass. 

You can only deactivate your pass until 23.59 CET the day before the start date of the validity of the pass.

What date is your planned travel with the Eurostar? Have you checked if there are 2nd class pass holder seats available from Lille, Brussels or Amsterdam?

I already activated the pass and the start date was 24th December which has already passed because I was going to travel somewhere but ended up not going so i haven’t used any of the 4 travel days. I am planning to travel on Eurostar on the 7 Jan 2023 and already checked other departing station and there is none available :((((

There is availability from Lille Europe to London on the 7th of January. 

Yes they check your reservation and pass onboard the train as well as on entry to the E* area at the station. Very similar to getting onboard a plane where the onboard team double check you are boarding the right plane. On the 2 occasions in 2022 when we travelled E* they accepted just the QR code on the reservation for entry to security, passport and waiting room but checked both pass and reservation onboard.