Masters Student in Math: Picking an Advisor I'm a masters student looking to graduate next school year. I've picked a masters adviser already, who's doing some things in number theory but more on the analytic side (not quite my cup of tea, but close enough). I found out recently that there is someone doing algebraic geometry in my department (which is what I ultimately want to do when I pursue my PhD). Would it be in bad taste to switch advisers? Nothing with the original adviser is set in stone, just a verbal agreement and he gave me a paper to start reading.

Also, more importantly, is the closeness of a masters project to what you eventually want to be studying all that important in terms of applications to PhD programs? That is, if I do a project in something I'm not going to pursue further in PhD, would that look weird to PhD programs I'm applying to? The reason I ask is that the professor doing algebraic geometry at my school is not a very friendly guy. Thus I may not be able to get a good rec letter from him, let alone have an easy time working with him. 

[Just for some perspective, a masters project at my school mainly consists of reading a paper by someone else, studying it thoroughly, and then presenting it.]
If nothing is set in stone yet, it would be reasonably to explore the possibility of switching advisor.

However, I don't think it's necessary. You are not expected to be doing exactly the same thing for a masters as a doctorate. Or indeed, even anything massively close. Within the UK at least, many doctoral students will come through the MMath rather than a stand-alone masters, and in theory may not have done any sizeable project. My project was completely unrelated to my thesis topic.

Indeed, personally I would suggest not switching topic. The way you are describing things, the topics are close enough that having the additional breadth will actually help your doctorate. You should develop an understanding of material around your thesis topic, not just the exact question you study.

But also do start talking to the potential doctoral adviser, and start to get a feel for whether the two of you are happy to work closely together.