You can apply for desired program you want, without changing and get a certificate qualify you for good career . 

When I did my PhD, I was teaching full time in a secondary school.  I was also caring for an aunt who was 79 years.  My time was tight but I attended several conferences with another friend who had a scholarship, and didn't need to work.  We paid the fees for conferences but it was affordable.  (At that time, we already found that COLLABORATION was the alternative because not 1 person has all the resources.)

I do not think that changing fields in the short time between PhD and postdoc is feasible for some of us.  Now perhaps with easier mobility of goods and services, we can look further afield to universities in other countries for a postdoc, if our circumstances and family responsibilities allow. (Sribas, this is a good Q.  Perhaps you can find more appropriate topics to place it.)

This is a good question.

The short answer:  It is the supervisor that decides whether to accept a new Ph.D; (in a different field) as a postdoc in the supervisor's research area.    This would be an unusual situation, since the whole point of a postdoc is to extend knowledge gained while completing a Ph.D.

As far as I know, postdoctoral position is a temporary appointment of a person who received a PhD degree, to strengthen his/her academic depth and research capabilities displaying growth through publications and cooperation with senior  faculty in order to be able to get a faculty position.

I am not sure about changing fields during postdoctoral studies, as changing  requires equal if not more of knowledge and certification of PhD degree as well.  With out having a PhD degree in a field there will not be what we call postdoctoral in a filed. But the process may be different in some countries which I am not aware of .

After PhD, one can go for postdoc either in related field or interdisciplinary field.

Each 'individual-education environment' interaction is unique implying that transitions between PhD and PostDoc are individual-specific?

Is the goal to become a researcher (e.g. becoming more specialized) or to become a professor (e.g. becoming less specialized)?

There should not be a big problem. PhD is the evidence, that the student did understand scientific methods. After PhD time, the new born scientist should not stop to learn and research. If using the learned and experienced techniques, I´m convinced, the post doc can successfully research in a differing field.

Dear all,

I went through this experience before, I'm from a science background and found my self enforced to work on engineering researches. I admit its not an easy to change the specialization specially at the beginning, but you need to be more systematic in this case and to collect knowledge and tools as much as you need for your proposed work this may take the first six months from you, so you need to be careful and inform your supervisor with each step you do. But if your supervisor is not from the understanding type I advice you not to do it, because it might be a dangerous adventure.

Dear @Marcel, it is a fact that in some of our (developing) countries, some persons with a PhD cannot find a job that is exactly their field.  Universities do not promise to get a job for students.  Some PhD persons are earning the salary of a BSc holder.  You may not even understand this, but it's true.  The only solution was to work in other countries.  You can see so many global citizens on RG platform.

Education in human versus non-human sciences: why is it easier to switch education/research when only non-human species are involved?

Students from non-human sciences usually receive university formations lasting three to five years, occasionally followed by a PhD formation lasting maximum two to ca. four years. Students from non-human sciences rarely or not continue to work in the same field with the same model system or model species after the end of Master or PhD formations. By contrast, education in human medicine might take at least seven years, with the additional option to focus for years on one aspect of the human body (e.g. one organ or body part) or pathology (e.g. allergy, metabolism) to become so-called ‘medical specialists’. Perhaps true specialists in nonhuman sciences should limit research (Master/PhD level/PostDoc), and eventually education, to not more than one or two model species or a single aspect (e.g. color or vision) of a limited number of model species or systems? Some people claim it might require 10 or more years to form a scientist, but then this will obviously depend on how many different topics, model species or model systems the individual scientist is willing to cover.

Why is it easier to switch education/research when only non-human species are involved? The answer is that humans do not want to take risks when research on humans is involved?

My personal opinion is that it depends on the gap between the fields. If you can mix your current field with the new field, it becomes less painful to do the switch. As fields diverge more, the less likely the outcome will be a smooth transition.

Here is the similar thread at Academia. Transition from PhD to postdoc with an intent to change field! Fine pros and cons are given in introductory part. 

http://academia.stackexchange.com/questions/7997/transition-from-phd-to-postdoc-with-an-intent-to-change-field

Based on the openings available both specific field and inter disciplinary is also possible.

https://www2.le.ac.uk/colleges/artshumlaw/documents/m3event.pdf

Dear all,

Good day

 I think it's easy to switch your domain by mixing your current field with the new field, because it is difficult to completely change the fields in a short time for some of us .

Regards

Please consider the Stowers Institute for Postdoctoral training.