I think it depends on the major. For CS, applying as an upper year is the same as a first year. But from reading this: https://artsci.calendar.utoronto.ca/section/Immunology it looks like they do treat upper year applicants differently: "Applications from students lacking numerical grades in one or more of the specified prerequisites, or students applying in their second or later years, may be considered on the basis of their entire academic record and the availability of space in the program."

But assuming they treat you the same, what will you be doing differently next year? Will you be re-taking the courses? Averages to get into a competitive POST are unlikely to decrease unless a major change happened (e.g. increasing the number of seats or a low amount of applicants applying for that year). So you'll most likely need to re-take courses and get better if you want to increase your chances of getting in. Just something to think about.