Anything is possible, and it is up to the 'elite' JC's principal
to decide if he/she is willing to admit you in. In other words,
case-by-case interview basis.
But one valid concern the new principal and HODs will have, is
that your previous JC would have covered different topics for
all subjects. Eg. for Chem, some JCs complete OC (Org Chem) in J1,
while other JCs start OC only in J2.
But apart from that, there is also the dubious issue of
'loyalty' and 'elitism'. Both JCs (previous and new) might frown
upon you for wanting to switch JCs just to get yourself a nicer,
more 'elite' JC brand name on your certs. It's like you're not
loyal to your company, you scream "every man for himself!" and jump
ship at the earliest opportunity. Both old and new bosses (and the
industry in general) might frown upon your behaviour. (it's similar
to, if a girl can betray her current bf behind his back to
have be with u, she can also do the same with u once
she's ur gf).
You're probably doing this thinking it can help you get into an
elite Uni course, eg. Medicine. In fact, your interviewer for your
elite Uni course might also frown upon your behaviour, meaning your
plan in effect backfired.
And really, it doesn't matter which JC you're from (eg. RJC, MI,
private candidate), it's the quality of the 'A' level cert
that matters. If you're the boss, would you discriminate against an
employee from a 'lousy' JC, or poly, or ITE? There is no right or
wrong answer, simply a matter of ethics. Do unto others as you
would like others to do unto you.
In any case, if you're sure that's what you want, go ahead and
request for interviews with the principals of 'elite' JCs, after
you obtain your stellar Promo results. You may or may not
succeed.