How easy is transferring majors?
I think it's going to really depend on the school, when you decide to transfer, and which majors you are transferring to and from. At least at my college, transferring before the second year (from one design major to another) is relatively easy and wouldn't result in having to take any extra classes. After that it's probably possible to transfer, but would most likely require taking an extra year or two of courses.
It's going to be much more difficult if you're switching between a non-design and a design major. At my college there is no preference given to applicants who are already part of the university. They have to apply through the exact same process as non-university students and face the same odds of getting in (roughly 10% at NCSU). If you don't get into the design program at the college you want DO NOT go to another program at the same university with the intention of transferring into design later. You'll be much better off building your portfolio at a less selective design school and transferring in to your original choice from there.
I haven't read all of the responses so I'm sorry if this repeats anything:
I'm a senior at the NCSU College of Design in Raleigh, NC. I'm double majoring in Graphic Design and Art & Design though I only plan on completing the GD degree. I transferred to the design program at NCSU from UNC-CH where I was majoring in Bio Medical Engineering. The design program here is comprised of 4 years of studios, so even though I had some general requirements filled it will end up haven taken me 6 years to get my degree.
For me NCSU was the only real option. It's not the best school in the country, but it's in the top 10 for my major, and has one of the best programs for a public university. Because it's a public university and I'm in state it will end up costing a fraction of what out of state or private school tuition would have cost. 4 years at ncsu will end up costing me less than one year at risd.
I think what you get out of college, especially in design, is going to be mostly affected by what you put in and not what specific college you go to. In my field an NCSU degree still looks pretty good on a resume, and when applying for design jobs the university you got your degree from is less important than the quality of your portfolio. Would a RISD degree look better on my resume? Probably. Is it worth an extra $100k? That's a question you'll have to answer yourself.
One other thing to keep in mind when you're researching schools is whether or not they are only design schools (like RISD) or are part of a larger university. At NCSU I have a lot of general college requirements (science, math, english, pe, etc.) that I have to take. Smaller design only school may not require that and certainly won't offer as large a variety of non-design classes. Personally, having a well rounded education was very important to me. It may or may not be for you, but it's definitely something to consider.
Finally, and keep in mind this is entirely anecdotal, I've found ID majors and Architecture majors to have very different personalities. Of all my architecture friends and ID friends it's hard to imagine any of them particularly enjoying the other major. I have a feeling you'd know fairly quickly whether ID or architecture is the right major for you.
I'm taking an ID studio right now and I'm really enjoying it. If I had to do everything over again I probably would have chosen ID over Graphic.
P.S. If you happen to visit the NCSU College of Design website please don't judge the college based on its appearance. They took down our old, but at least aesthetically pleasing website and replaced it with the current one.