Unsoldering the FireWire port pins on an external hard drive's green board and replacing them with a USB port is not feasible due to significant differences in signal types and communication protocols between FireWire and USB. FireWire uses two differential pairs and has different signal impedances compared to USB's single differential pair. Instead, using a USB to FireWire adapter or a FireWire add-in card is recommended, though compatibility with laptops may vary. For a more straightforward solution, purchasing an external USB hard drive enclosure to house the existing hard drive is often more cost-effective. If parts are available, one could theoretically attach a USB-compatible green board to the hard drive, but this requires specific knowledge of electronics and soldering skills.