Is there any risk in applying for a different graduate program, while already doing a PhD? I'm doing a Ph.D. (in Europe) in field X, but my interests have shifted towards a different subject (say Y) over time. I've tried to shift my research to be on interdisciplinary topics crossing the two areas, but it's far from satisfactory for me and still my advisor constantly pushes me towards research projects I'm not really interested in.

Initially, I thought I'll finish the Ph.D. in X, develop some transferrable skills, and then do my postdoc in my area of interest, but my advisor is limiting me to opportunities (collaborators, conferences, projects, etc.) within my old field of interest and I just can't imagine an academic career in X for myself. I want to stay in academia, but join a Y department, which would be impossible without a proper training in Y or substantial research experience and networking with the researchers in that field.

So, I'm now considering applying to grad programs in Y, but I actually have no idea about my chances. I have some interdisciplinary research experience and minor collaborations with researchers in Y (plus self study of course), which is what I'm relying on for my application. I'm also getting recommendation letters from my collaborators working in Y, who are aware of my situation. But, since I don't know whether I'll actually get in or not, I don't want to burn bridges or make my advisor angry or disappointed. After all, my only chance for pursuing my interest if I don't get into the grad programs I like, is to finish my current Ph.D. succesfully and find interdisciplinary research positions (which comes at the risk of being a "research associate" or some sort of "life-long postdoc" and never getting a tenure). My alternative is to finish my Ph.D. in X and then go and do a masters or possibly a second Ph.D. in Y (which is suboptimal, but still allows me to follow my interests), and I don't want to burn bridges for doing that either.

This means I'm applying to grad programs in Y, without telling my current advisor about it. I'm worried the department I'll apply to might (possibly unintentionally and just in a coffee chat at a conference) might reveal this to my advisor or other members of our group, especially because in a few of the departments I'm targeting, there are people who actually collaborate (or have collaborated) with our group. I was wondering whether this is something I should worry about. Are there professors on the the admission committee (for master's programs in Europe)? Am I overthinking this or is there a real chance that this could happen? Would a reviewer ever email my advisor and tell them "this student of yours is telling us they don't want to finish their PhD and want to move to our department and do a master's in Y and then do a PhD in Y!"?!

++ Please note that areas X and Y are traditionally considered completely unrelated, but there are modern interdisciplinary research combining practices of the two. That being said, it's still almost impossible to get hired at a Y department without a formal training in Y (especially if you have a Ph.D. in X, the general attitude in Y communities is that you don't really understand Y).

++++ By grad programs, I mean master's programs (in Europe).
In most cases it would be foolish to hide it from your advisor. But I don't know them or their personality. Some people would react very badly and others would be very supportive, with every possibility in between. For a vindictive person, you need to be extra cautious. But those folks are fairly rare. 

One issue you should consider is whether you are leaving others with problems to clean up after you go. If there is none of that, then people are more willing to be supportive since it is easier. But you may have control over when you go and the state in which you leave any joint research. 

The reason it is foolish is not just that the person might try to torpedo your career (possible - unlikely), but that they may have options for you that you haven't considered. Those options might involve X or Y or a melding. But they've been around longer than you and may have a much better idea about what is possible. 

But this is a sit down face to face sort of conversation. I wouldn't express it in terms of like and dislike, however, but that your recent studies are pushing you toward one thing rather than the other and you want to continue those explorations. 

Perhaps, knowing the situation, the advisor can make room for it to happen. If not, you can, perhaps, leave with good mutual feelings.