Not likely mate. Being from old soldier batch during NS days and
then "new Army" during reservist days, I have observed how
training/ courses being doled out to soldiers.
During NSF, 99.99 % unlikely to be able to choose your course, much
less yr vocation (it is fated, as some cynical friends said). The
only way you can "choose" is like this.... at recruit stage do well
in PT, SOC etc, be as cooperative and show leadership signs. You
then get picked to OCS or SISPEC. In OCS or SISPEC, do well in say
your presentation and orders giving, but keep to being average in
PT... high chance you get selected to be an instructor somewhere.
When you become instructor, be a good one... likely to go to a
number of courses/ attachments (but certainly not airborne ). You
will find out what I mean. This was my career path !
In reservist... well , another matter. I have seen friends
retrained from signaller to combat medic. From infantry to combat
engr. Even stranger retraining have taken place (but can't mention
as sensitive). There are conversion courses. But this depends I
believe on your post NS education, the manpower requirements in the
reserves, and of course your last NSF posting before ORD.