I want to buy an LG Optimus One prepaid but it's with Telus. Can I just change the SIM or do I have to unlock?

The LG Optimus One goes for $128 at my local Wal-Mart, which is the prepaid price. The unfortunate thing is that I have to buy it under Telus. I've don't know much about SIM cards, so my question is, can I just pop out the Telus SIM card and insert my Rogers SIM card I have in another phone? Will it work or do I have to purchase one of those 'unlock codes'?

If the LG Optimus One is locked with Telus network, you need to unlock it. I suggest you can approach Unlocking4U.com and specify the country,locked network of phone. Based on the availability you can get the permanent unlock service along with free instructions.