So I have an iPhone 3G with Fido (15 months left on contract) and I want to

[just me xx]

upgrade to iPhone 4...? ..if I buy an unlocked (or locked to Fido) iPhone 4, can I just cut my current SIM card, with the tool, and pop it in the iPhone 4 and it should work with my current contract?

Sorry I'm bad with this kind of stuff.. thanks in advance