Final answer:
To replace a Firewire card with an 802.11n wireless network interface, power off the workstation, remove the existing card, and install the new wireless card in the same PCIe slot. Once installed, power on the workstation and install the necessary drivers for the wireless network interface.
Step-by-step explanation:
To replace a Firewire card with an 802.11n wireless network interface on a Windows workstation with no unused PCIe slots, you should first power off the workstation to ensure safety and prevent hardware damage. Once powered off, you will open the case, remove the Firewire card from the PCIe slot, and insert the 802.11n wireless network interface card into the same slot. After the hardware swap, you can power on the workstation and install the necessary drivers for the new wireless card, which Windows may do automatically or may require installation from the card manufacturer's driver software.
Before making any changes, ensure you are grounded to prevent static electricity from damaging the components, and handle the cards by their edges. After the hardware installation, verify that the new wireless interface is recognized in the Device Manager and connected to the network properly.