I am currently on my working holiday permit that expires in a month.
But i just sent in my application to be approved on the young professionals permit.
do i need to get a visitors visa in case my permit expires before i get accepted or can i stay as my application for another permit has been applied for?
And if accepted do i need to leave the country and come back to get it started or does it just keep going when accepted?
You need to apply to extend your stay as a visitor ASAP as there is no implied status that I am aware of for the IEC program transitioning to a different IEC category. Plus of course you will need to stop work once your current WHV expires. http://www.cic.gc.ca/english/visit/extend-stay.asp
Once your YP is approved other than leaving the country you will need to flagpole at the nearest US border and meet any conditions again such as insurance/funds to activate you new work permit.