Should I change thesis topic? I'm a student in the technical field A in a European university. I'm currently doing a master thesis in a field that's notoriously at the interface of subjects A and B, I haven't officially started the thesis time yet, but I've been doing preliminary work for about a month and a half and there is verbal agreement with my supervisor that I'm doing it.

The more I delve into the thesis and the more I realize that, while being at the interface, the topic is mostly subject B, with very little subject A.
While I don't hate subject B, I've come to realize that I'm not really interested in it, and I'm finding working on this thesis rather demotivating.

I still could technically leave the thesis, although I suspect it won't make me look good in the eyes of my supervisor, since I already told them that I intended to go through with it and I've had multiple occasions to leave by now.

On the one hand, I still like other aspects of the interface between A and B, doing A with a B perspective instead of the other way around (which I'm doing right now) is still a possibility for me, hence I don't think it would be wise to break ties with my supervisor and their group. I feel that if I leave the thesis I'm just going to look bad, lose potential references and just restrict my possibilities for the future. The group is quite important in the subject and the master thesis admission process is pretty selective, and I don't have any other readily available alternatives. From this perspective, it seems better to soldier through this thesis and do it well in order to have good references. I've been told that it's not at all uncommon to do a PhD in a different subfield than your master thesis. And who knows, maybe interesting elements of subject A (or even B) might come up along the way, though that seems unlikely to me right now.

On the other hand, I fear that committing to a thesis in a subject I'm not passionate about will prevent me from doing a good job. I fear that I'll just survive through it doing the bare minimum and end up with a mediocre thesis and mediocre references in a subject I don't really enjoy, making it much more difficult to both stay in this subject or switch. Beginning a thesis I do not enjoy, while I have still time to leave, feels somehow like lying to my supervisor, and starting a project alreading looking forward to end it is not a good looking prospect.
Even if I do manage to write a good thesis, I fear its strong element of subject B will make it more difficult for me to be considered for subject A positions.
Also I would like to stay at the same university after my Master's, and I think the chances of that would be made better by doing a thesis on a subject I like, which could be continued in a PhD.

I guess I'm demotivated and looking for advice from experienced researchers on what of the concerns I listed are sound, and what, if any, am I exaggerating.
This is certainly a question you need to explore with your supervisor. What they would accept or recommend is certainly important. 

What an experienced researcher would do is one of the following, maybe both. First, they would be likely to consult with colleagues (not unlike your supervisor) and explore viable alternatives. 

But, assuming that they have a secure appointment, such as tenure, they would do whatever most appealed to them. Of course, wisdom would suggest they keep their options open. 

But you don't have the luxury of a secure appointment. Your supervisor's advice is needed and should probably be followed. They will also have advice on the probably effect on your career, if any. 



Actually, I think you may be suffering from a common ailment: Buyer's Remorse. For most such situations, the feeling of regret goes away in time. If you are making progress toward a better future, you are probably doing the right thing. Don't let emotions get in the way of progress (easy to say, of course).