Given it's already Week 6 of Term 1 technically, I reckon (from a teaching/admin point of view) there's only a very slim chance of the change (change referring to picking up a new 3/4 subject) being approved. Schools approve/decline changes based on timetabling and blocking and given this is set well in advance, it will only be approved if you a) have a very good reason for the change and b) whether your change fits in with their predefined timetable blocking (as any major change will require a massive reshuffle of the timetable and of course they won't do that).
From a VCAA point of view, VASS allows the enrolment/withdrawal of units up until 6 May so from a technical perspective it is possible to make the change still, however schools need to finalise data by 25 Feb (which has
just passed) so that planning/prep can be done for all the assessments, auditing etc.....
You might be able to just
drop one of the subjects now (if that is something worth investigating?).. that won't impact anything significantly (unless it then means you don't qualify for the VCE)
tl;dr will only approve the change if it benefits them and you aren't just doing it because you've changed your mind
Sidenote: Not just pulling these dates out of my ass either, see admin advice:
https://www.vcaa.vic.edu.au/Pages/schooladmin/admindates/index.aspxGood luck.