Most of the userspaces are just compiled for ARMv6. If you've got your firmware (bootloader, kernels, devicetree overlays) up-to-date, then you should be able t

Most of the userspaces are just compiled for ARMv6. If you've got your firmware (bootloader, kernels, devicetree overlays) up-to-date, then you should be able to boot the OS on any Pi (the bootloader detects hardware version and inits the right stuff, including loading kernel7.img for the Pi2/3, instead of kernel.img for the 0/1/1+ models). Ubuntu and some of the other 3rd-party OSes are compiled for ARMv7, and won't run on the 0/1/1+. A couple are compiled for ARMv8/AARCH64, and won't run on anything but a Pi3. There's a lot of work that went into making things like Raspbian as close to foolproof as possible.