porting phone number to Dell voice from Rogers

[avengine]

I have a cell number in Rogers, and my contract is expire on Dec 4, 2012, I know that I can contact Rogers 1 month ahead to cancel the monthly charge. But I do want to port the old number to Dell voice to keep it, and Dell customer service told me it will take 2-4 weeks and cost me $25 one time cost.
My question is what is the best timing to make the switch without paying another month of $35 to rogers again. (I hope).
I know that I have to make the switch when my number is still active in Rogers account, but if I cancel the account too early, Rogers might charge me $100 penalty, if I go over 1 extra day, Rogers will charge me another $35. what kind of business they are in...

anyone has this experience and what is the best way to do this?
thanks.