There is no clear-cut answer to this question and the rules don't say anything.
Several options are possible:
1. Changing Specialties can be done between terms. Just do it.
2. Changing Specialties requires a new Qualification roll (with or without the DMs, Ref's decision) to change, but if you fail, you just stay where you are.
3. Changing Specialties is like changing careers and treated as such.
Personally, I use #2 with no age/term DMs.
Looking at the Sample Characters from the various MGT books, they don't have anyone change Specialties, or even try to change Specialties, so they are no help.
It is really up to you and how you think it should work. In the Military, it is difficult, but not impossible to change your Specialty, where as in Civilian life, it seems to be easier = you get a new job within the same career.