It depends a lot on education -- especially on education specifically aimed at strengthening the planned new type of work.
In terms of employment opportunity, I think it helps a lot to be able to demonstrate some sort of self-education or coursework aimed at the desired new type of work.
Anyone with a bachelor's degree in engineering from a school in a technically advanced country should be able to continue learning and switch from a habitual role to a new and different one.
Being able to switch gears from time to time over your career is not only possible, I think it's mandatory. To say an engineer is unable to take on new tasks that are different from what he or she has done before is to say that engineers are incapable of learning new things after they leave school. Such engineers tend to not be very successful.
Well, it depends in which area you have been working. I started working as Test and Verification Engineer and I know that there are several paths you can take. If you know design details, you can become a designer/developer, if you interface with customers, you could take the Application Engineering path and so on,.... As it was said, it also depends on your personnal perspective and skills. I always think that an engineer needs to have the potential to switch gears because this is an extremely dynamic world.
In real life, engineers do switch gears and they are quite successful at that. Many time, potential employers are ignorant of what are the capabilities of an engineer. They themselves may not have worked in similar situation. They only appreciate engineers after they see the results. I always encourage engineer to take new type of problems or change gears totally.
This is one of those touchy subjects. As an Engineer who have switched once from IT to Embedded R&D; I have to say it's possible but I think it is more likely in earlier years of ones career. I was one of those that was stuck at one specialty for a good few years and know it well how you can get hooked to your initial expereince for a long time, however I also belive that "when there is a will, there is a way" off course the possibility of this switching becomes lower by age, we have to consider there is a big risk involved with any major career switch which one has to consider first.