Changing specialties - how difficult is it?

Hi everyone. I'm new to this board, and can't quit reading all the great posts. (What a fantastic resource!) I'm switching careers, start an accelerated BSN program in about 2 weeks, and have what is probably a naive question. But I'll go ahead and ask it anyway: how easy or difficult is it to switch between specialties once you take your first job out of school? Are people pigeon-holed pretty quickly? I ask because, for me, one of the attractions to nursing (besides the more obvious ones) is that it seems to be very broad, with one educational pathway giving you access to so many different types of jobs. I've always been somewhat restless (professionally, that is!), and usually feel I need to make changes every several years or so, move on and be exposed to new things, etc., so... would someone who wanted to sample lots of different specialties have any problem doing so? Do people do this typically, say, something like med surg for a few years, then ICU for a few years, then ER, etc.?? Or are there some barriers I'm not aware of which prevent this kind of lateral movement (certificates, specialized training?) Does the possibility vary by region, urban vs. rural, etc? It seems like the so-called nursing shortage should make this somewhat possible, but I just don't know how much the tasks, experience, etc. overlap. I'm worried because a lot of my classmates say they already know their specialty, while I have only a vague idea.... (Maybe Med Surg, maybe ICU. I know I'll get a better idea after clinicals!) Anyone have any thoughts, advice, or suggestions on this? Thanks!