Voluntary changes of trade are possible if:
a. the trade you are in is "Open" for transfers out,
b. the trade you want to go to is "Open" for transfers in,
c. you meet the entry requirements for the new trade, this may require interview by the Personnel Selection Officer (PSO, and even retaking the CFAT if you need a higher score than what you have on record, and
d. there's nothing on your file that may restrict your transfer such as unresolved medical issues, administrative proceedings such as Counseling and Probation, etc.
If those conditions are all met, then you can be considered for transfer to a new trade. Some trades fill vacancies with trained soldiers to make up for losses in training of recruit applicants (or other unforecasted attrition) for their own trade, but numbers are rarely consistent. Some trades take very few transfers from other trades and people have been known to apply year after year before being selected (if at all). You would have to renew your application each year for consideration.