Please note also it will also depend on regulations in place. I'm not sure army rules but I know with NAVRES that the following are requirements for a VOT:
1) The occupation you're joining must have openings;
2) The occupation you're leaving must NOT be in the red (i.e. under strength);
3) You must be trade qualified in the occupation that you're leaving.
There's nothing wrong to speak with your unit Chain of Command to determine if it can be entertained, but realistically likely won't be considered until you're done Basic (at least from my experience when I was the Chief Clerk of reserve units)