ya, i don't know about doing it before knowing you have a SSHRC (it might make things complicated and/or raise unnecessary red flags if they think you're changing it b/c of something related to eligibility?), but I switched my start date for my MA SSHRC as I described above (I applied as a MA student, not as an incoming undergrad too). SSHRC sent me an email saying I got the award, and when I responded to confirm the acceptance, I also asked to switch the start date.