ethics - Is it unethical to interview at a more prestigious program while currently enrolled in a PhD somewhere else?

I applied at two different PhD positions at universities X & Y a while back (both in EU, I prefer not to disclose identity for specific reasons, sorry).

I got into X and have been here for about 2 months, and I heard from Y a few days ago inviting me to interview with them. Y is more prestigious, and also many of my friends are there. (Note: Both positions had different starting dates, X was more of a start-earliest program, Y is for 2015 admits)

Considering I am more interested in studying at University of Y, would it be unethical for me to take up the interview with Y, after I am here for 2 months? (I am at X as a RA presently)