Hi Lily - you've gotten some great advice so far, here's a bit more to consider.
First, I assume your college offers Architectural Engineering as a degree. If so, compare the coursework requirements, and see how they overlap. Architectural Engineering is a relatively new discipline in the USA, and my understanding is that it is a combination of structural, mechanical and electrical engineering.
Next, do the ArchE courses sound appealing? Are the required classes ones you will enjoy?
Third, your school may have ArchE options that allow for electives in different fields, if so, I'd suggest adding (or using) your MechE coursework in fluid dynamics, HVAC or fire protection, if offered.
So that's my thoughts on the college degree.
To answer is ArchE more limited than MechE? Absolutely. You are going to find far fewer job postings for ArchE than MechE. So if you are concerned about future employment, stick with the MechE degree and focus on coursework that pertains to mechanical systems for buildings -- fluid dynamics and heat transfer as fundamental physics courses -- and then upper level electives that involve fire protection, hvac, etc. And if you haven't had it yet, a CADD class or two. That way you can go work for an architect, or you can be employed by a firm that builds industrial facilities of any kind or as a facilities engineer.
However, your college may have a phenomenal ArchE program, and has a 100% placement for all its grads -- check with your career placement to see what they can tell you about job placement historically.
Also, look for summer intern opportunities in MEP (mechanical, electrical, plumbing), in addition to architectural engineering for 2019. That might help you decide too.
Good luck, and let us know what you decide!