Hi,
USMLE MEDICAL license preparation course of less than 6 months, which is NOT a valid Canadian post secondary course or from a valid Canadian recognized post secondary educational institute resulting in a certificate, diploma or degree. And hence you CANNOT change your Study permit just to study this USMLE prep course.
Remember your Canadian study permit is for 2 years. This means..even though you should be legally studying that diploma, one can always choose to do other things...and many people often work..ignore studies..but all at their own risk. So unless caught...one is entitled to legally stay till their study permit expires.
From, what I see... now you have 3 options:
1. If you legally leave your current Canadian course /declare... then you CANNOT study and stay in Canada..on that study permit... Go back to your native country and prepare for USMLE. Not reccomended.
2. Since that USMLE course prep is just 3-4 months long, you should continue your Canadian diploma (and prepare mainly for USMLE simultaneously)... so, just incase you don't get to clear that exam... you can always complete the diploma... get 3 years PGWP... try to get Canadian PR.. Reccomended and Logical option.
OR
3. If you choose to simply ignore and drop your current semester/term of your diploma and prepare full time for USMLE, and later decide depending upon your USMLE results..whether to continue your diploma studies or not.
So, you need to take an informed and wise decision.
Best of Luck.
***All opinions expressed are purely personal, and pieces of information mentioned are to the best of my knowledge... You should consult other sources too.