In my batch, there was an Indian guy in my section whose
e-posting order stated that his vocation is a medic and he will be
attending the course in Nee Soon camp. In fact, when our platoon
met up for a gathering nearing the end of the 1 week block leave,
he himself told us that his vocation was a medic. He was VERY
disappointed as he made no secret that he badly wanted to enter
OCS.
However, his vocation changed and he actually went on to OCS and
last i heard, he ORDed as a LTA. The Indian's uncle is a COL (some
more he is a COL 2, assuming an important position in
Mindef.)
Naturally, rumors were flying that his Uncle pulled him to OCS. How
true that is, i won't know. But that is definitely one example of a
person switching vocation out of the sudden and entering OCS.