Is it unethical to interview at a more prestigious program while currently enrolled in a PhD somewhere else?
No, there are no ethical issues. There is nothing unethical neither in keeping it confidential until you are officially admitted at University Y (and I think it's advisable).

The only not-so-good thing could be that if you built a relationship of trust with your current supervisor (but I suspect you haven't since it hasn't been long), this person might get disappointed to hear you leave. But generally, people understand the argument of higher reputation of the program.

Consider however, that University X might not reimburse the tuition you already paid and that you might encounter other administrative inconveniences.