Transferring to a PhD program I had been previously admitted to I will be beginning my PhD (in a field that's halfway between social sciences and humanities) in the fall. During my application cycle, I was admitted to all three of my top choice programs, and was heavily recruited by all three - to the extent that when I turned one of them down, the admissions committee chair emailed my undergraduate advisor and asked her to try and change my mind. Eventually I committed to the school that I thought was the best match for my research interests, with one professor in particular I was extremely excited to work with and who was excited to work with me. This choice was a bit of a gamble- while the research going on here is probably the most exciting in my subfield at the moment, the program itself is pretty solidly second tier (as opposed to the two I turned down, which are definitely top 5) and fairly narrow in its focus (i.e., if my interests shift even within my own subfield, I'm not sure the department will be able to accommodate that).

Even with the potential risks of this program, I decided the research was fascinating enough, and I wanted to work with this professor and his lab enough, that it was worth it. However, since committing I've heard some things about the organization of the program as a whole, and specifically the advising styles of this professor and a second professor I would likely want to collaborate with, that really worry me. Also, I've had some conversations with my future advisor that similarly make me think that it may be a bad fit in terms of our working/learning styles. Basically, I'm afraid that I chose the wrong program for me and that I'll end up unhappy, unproductive, and (in the end) unemployed.

My questions, then, are if, after a year or two in this program, I feel like I have a bad "fit" with my advisor, am unhappy, and/or my research interests have shifted so that they will be better served by one of the other programs:

1) Is it generally possible to transfer, assuming that my academic and research performance at this PhD program are still very strong?

2) Is it possible that one of the programs I turned down in the past will accept me again? Or would the fact that I turned them down previously make them reluctant to do so, even if I made it very clear that I would definitely attend if accepted (I would probably only reapply to one university, so that I could say with confidence that I would attend if accepted)?

3) How seriously would this burn bridges with people at the program I'm currently attending, and the people they work with at different universities, if I did decide after a year or two that I wanted to transfer?
First of all...you haven't even started the program yet. Give it a chance. Go in with an open mind. Hearsay is unreliable and it's normal to have second thoughts about something once you've committed to it. So put the question of transferring out of your mind for now, and focus on getting fully integrated into your department and your program. If you're mentally one foot out, you might simply turn this into a self-fulfilling prophecy.

To more directly answer the questions:

1) Yes, it's generally possible; I have a couple friends who have "transferred" PhD programs. However, "transferring" usually just means reapplying to a new program; you usually have to start over. So if you've done 2 years elsewhere, it may mean absolutely nothing and you may have to retake all of your coursework. At best, you might get credit for one semester's worth, but you're probably not going to be able to pick up where you left off at the old program. Generally, you also need the support of your advisor at the Old Program. In other words, the advisor you are going to work with in the fall would need to write you a recommendation letter for the New Program. In some cases where the relationship has gone sour, students have successfully gotten letters from other professors in Old Program, but those professors need to be very skilled in explaining why the relationship went south (in a way that reflects positively on the student.)

2) Maybe. It totally depends on the department. If you were so good that they were recruiting you heavily, and you produced evidence that backed that up in your first year or two, they might be eager to take you again.

3) It depends on how you do it, and the reasons that you transfer, and how reasonable your advisor is. Most professors understand a transfer that has to do with a shift in research interests - if you came to study X with Professor Smith but Professor Smith leaves, or Professor Smith radically changes his research direction to Y, or you change your research direction to Y - explaining to your advisor that you have appreciated your time at Great U, but your research interests no longer fit the program and you'd like to transfer to Awesome U to work with Professor Jones in your area, usually engenders understanding.

In the not-uncommon case that your relationship with your advisor goes south and you're transferring not because of research but because of personality mismatch, that can burn some bridges. But they may be bridges that you have to burn if you want to retain your sanity. Only you can make that decision.

In the less common but still possible case that your advisor is simply a jerk and hates when his students leave for any reason, even a reasonable reason can set him off. There's nothing you can really do about that, but there are typically sympathetic other professors that you can turn to for support and recommendation letters.

And how you make the break also plays a role. These are obvious things, though - don't badmouth your advisor, don't badmouth your program, don't disparage your classmates or other professors. Stay positive and make the transfer about you, and not the program itself.