Generally getting a Masters on top of a B.Arch won't affect your job prospects too much. But I could see two exceptions. One would be if you're trying to get a position at a top 1% design firm and you wanted more design thesis credentials and more of a research thesis to apply with. The second would be your position, getting a specialty Masters for a specialty role. Unfortunately, I don't have experience in a firm that does historical preservation to speak to so I don't want to speculate.
My recommendation would be to either test the waters a little bit, see what kind of job you can get first. Second would be to get in touch with some firms that do historic preservation work and see how much they would value that in an applicant.
Generally from architecture firms, I hear them say that it's more about the candidate and their personality and soft skills than there credentials.
The other thing to factor in here is the job market. In the good job market, we've been in the last 5 years, it's so easy to get a job, there has been a massive undersupply of candidates especially in the 3-5yr exp range. So if you're in the 0-1 or 1-3yr range you could very easily get a job and a good one without a higher degree or other credentials. Even in a bad job market, once you get past that 3yr mark, it'll be much less about your degree and much more about your experience and your soft skills, personality and so forth. If you have done studio work in your B.Arch or done your senior project on historic preservation, that'll likely be enough. That senior project will set you above most applicants.
It's hard to say what it will be like in 1-2 years, but; from what I'm seeing architecture has not been impacted much right now but if government funding, especially state and local, is the source of a lot of historic work; I think historic preservation could take a hit as state and local government tax revenue will take a hit. Further, things like affordable housing will likely take precedent in funding over historic preservation so that could exacerbate the lack of funding.
I assume if you're considering this you've looked into historical preservation and have perhaps interned in it. One of the downfalls of a specialty degree is that it won't help you much outside of that specialty so I'd really make sure you love historical preservation and want to do it for the long term. From my limited experience historical preservation is a lot of reading, very drawn out processes, very tedious. For example, a lot of time is spent on determining which solvent to use when cleaning each material. That is a lot of what historic preservation is I believe. But I would think it'd be really cool to get a job with the National Park Service. It can be really rewarding to work on high profile projects. If you like doing library research it could be really fun. You get to do a lot of things most architects don't get to spend a lot of time on. But also look to see how common it is for the architects to do these things vs hiring consultants such as historians. If you like doing city council meetings and things like that I imagine there is more of that too.
One thing I can tell you from experience is that renovations can be a pain (vs new construction) the code research is a lot more complex and the project doesn't often fall inside the box that is the code. Renovations often also mean fitting things like sprinklers, Mech, Elec, Plumbing, fire escapes into buildings which they weren't designed to accommodate. Dealing with hazardous materials. (Usually, architects aren't involved in that too much).
I hope that helps!