Of course if you apply Diploma, all the documatation are needed. This program is a good choice if you would like to switch to CS. Also, it is really competitive.
First, you better have a bachelor degree, actually it is not enough, most diploma students have a Master or Ph.D.
Second, your GPA should be higher than average at least.
Third, if you have computer related work experience will definetly be better.
Finally, your C++ grade prefer be at least A-.
THis information all gether from my friends, only for reference.
1. GPA you can refer to the post "How to calculate GPA".
2. Your working experience will present in your resume and your reference letter when you submit the application form.
3. COMP 248 is the course for C++, you can check the couse outline by visiting http://www.cs.concordia.ca/~teaching/comp248/
I think this course is the only prerequest course.