Bought business class ticket, but now agency wants to change it to economy

About 2 weeks ago I have bought a business class ticket from Vilnius to Bangkok and back for 2 persons from booking agent at tickets.lt. Later that day I received a PDF with business class reservation.

Today I received the following email from them:

Dear Customer, Thank you for chosing our service.

Regarding your booking on our web site, we would like to inform you, that due to the technical reasons your ticket was issued in business class. But, in fact, your order mentioned a booking in economy class of transportation. So, we have two options for you. The first one is to leave your ticket as valid for use, but your class of transportation will be the economy. The second one is a refund of the amount for your ticket. Please, choose which option is more acceptable and comfortable for you. And inform us in the answer to this email.

Please, contact us in the case of further questions. We are always glad to help you.

I'm an American citizen, tickets were bought using American credit card, and as I mentioned I'm flying out from European Union.

I want to fly to Bangkok, but only if it is a business class. My question is, is there any law outlining these kind of situations? Is it ok for booking agent just forcefully cancel my flight? Anything else I'm missing?