PGWP timeline, complete study program and move from part time to full time

Hello everyone, I appreciate your help on this one.

My case:
I am about to finish my studies. My last final exam at the college is on April 30, and I expect to receive my completion letter around May 28. On that same day, I will submit my PGWP application.

Quuestions:

• From when can I start working full-time? (I am currently working part-time).
• My company will be increasing my salary and hours. Do I need to sign a new contract with them? Would an agreement be enough?
• If so, is it legal to do this the day after I finish my final exams?
• What legal implications would my company have in this case, or what would be the recommended approach?
• Considering these dates, what would be the correct timeline? On what day can I start full-time, on what day am I not legally allowed to work?, and on what day could my company legally offer me a new full-time contract?

Thank you for your help.