It depends on alot of factors, the most important of which is if he is in a "RED" trade. Red meaning they will let him in but won't let him out. the list of current red trades are:
"NCM RESTRICTED REASSIGNMENT (OUT) LIST: 00017-02, 00019-02, 00100-01, 00329-01, 00116-02, 00117-02, 00118-02, 00327-02, 00134-01, 00334-01, 00169-01"
quoted from the latest list from CFRG.
Translated means.... naval weapons tech, airborne electronic sensor op, Met Tech, sig op, naval electronics tech (all), FCS Tech, Mat Tech, Med Tech, Ammo Tech.
If your DH is any of the trades listed above he can't , this month, get out.
If he is not then he just applies by Memo through the Chain of command to see the BPSO to "discuss my career options".
The appointment will be made, he will go to 1 and possibly 2 meetings, they will interview and give their decision. A message is cut and he changes trades and waits for the next course. Of course it would help if he had a list of the trades that are open for reassignment during the month he goes for the interview.
If he is in Borden on PRETC he may wait a bit for an interview but he has a right to go and speak to the BPSO, everybody does. EWOT depends on the wait until his course in his new trade, if it is a long wait then maybe but I wouldn't push it, you ask too much you just may come back with empty hands. Change trades first then fill in the blanks later.
Trust me I've been there. Literally.
Hope this helps. ;D