Should I turn my iPad off to replace the MicroSIM?
If by "3g chip", you are referring to the iPad MicroSIM card, then no, you do not need to power off the device to switch it out. However, if you're referring to the actual 3G radio inside the iPad, my advice is to take it to an Apple store and have the repair done by an Apple technician, as operating on the device yourself is likely to void the warranty.