I want to switch my iPhone 3GS from my personal carrier to Movistar! Who can help me? Ideally without using the IMEI.

You need to know if it's an "old bootrom" or a "new bootrom". If it's "old", you need Redsn0w and to install the iPad 6.15 baseband. Then, jailbreak it and install "Ultrasn0w" from Cydia to unlock the iPhone so you can use it with any carrier.