Regarding booting from an external USB device ...
... you can change the boot order in the "raspi-config" terminal program, and just leave the micro-SDcard in place.
Just be aware that the Pi will take around 30 seconds to boot from the internal micro-SDcard when it doesn't find an external USB Drive to boot from, if choosing the "USB boot first" option.
You can use this setup to either play with a new OS, or just to have a backup OS ready to go if one fails.
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Pete VK3PYE
Peter VK3PYE