Hi,
Great questions to think through before diving in — answers below.
#1 You can use your existing SD card, no need to switch. One thing to be aware of though: removing the USB Adapter is the method used to trigger a factory reset, so once you switch over to USB it defaults to that. If you’d like to swap back and forth between USB and non-USB WiFi, I can point you to the file you’ll need to edit to disable the reset script (covered in #3 below).
#2 By default, the USB Adapter broadcasts its own WiFi network (PiFi 4 or PiFi 5) and takes over that role from the internal wireless. Your wired connection to the router is still what gets you online, so that remains the fastest path. A nice side benefit is that since the internal wireless is no longer tied up broadcasting, you can also pair with other WiFi networks if you want — though wired will still outperform it. The main upgrade is much better broadcast quality from the USB Adapter compared to the internal radio.
#3 Going from your current config to USB is super easy, but as mentioned, removing the USB Adapter is designed to trigger a factory reset by default.
If you have keyboard/CLI access, you can edit /etc/sysp/pifi_factory and either remove or comment out the line:
firstboot -y
That’ll let you swap between USB and non-USB as often as you like without disrupting your config — you can unplug the adapter, swap SD cards, or leave the adapter connected, whichever you prefer