Here is the requirement to change trade
To be eligible to apply for AVOTP, the member:
Must have completed 48 months of continuous service by 31 December of the year in which the application is made.
Must meet appropriate medical category for desired MOC
Must be QL4 qualified in current MOC
Must be Pte/Cpl or if MCpl or above, must be willing to relinquish rank and pay to Cpl
Member’s occupation must be "open out" as detailed in competition message
or
compulsory occupation reassignment is necessary in the interests of the Service;
a recruit has failed the occupation training course for the assigned occupation but is recommended by the training agency to be suitable for possible reassignment to another occupation;
a change in medical classification necessitates a change in the assigned occupation but does not require release action; or
a recruit, having sufficient knowledge of the occupation training to date and, for substantial reason, requests occupation reassignment.
There the site with all the info (it from my base but it all the same )
http://edmonton.mil.ca/1asg/asu%20web/bpso/bpsohomepage.asp