hey katie_batt. I know of 3 ppl that applied after year I and I know that 2/3 of them got in. You have to maintain a 9.5 average during first year and also submit a supplementary application again. It really depends on how well you do in first year. Its a new environment and learning style, so your marks may drop a little or a lot. It is possible to get in during second year, if you work hard. I can't really commit that much on the possibility of getting a 9.5 average, cause everyone has different learning styles, etc.
Here is the site that has more information:
http://www.fhs.mcmaster.ca/bhsc/admissions/level2.htm
There is only the Year II transfer, after that you can not apply to transfer in again. Also make note of the course requirements, but genreally you have most of them if you are in sciences. hope that helps.