Android root and custom ROMs/firmware
Of the phones that I have had, I've loaded custome Android roms on a few of those phones, starting with the first Android phone released (HTC Dream, with the 'Jay Leno' chin), the HTC Desire, and the Sony Xperia M4 Aqua.
Normally whats required is that you have to get root on thsoe phones, which allows you acess to the bootloader, in order to load a custom ROM.
Before Cyanogen became the biggest ROM around, I used a few others on the HTC phones (Rosie is the one I rememeber).
The latest one I had to do was upgrade the version of Android on the Sony M4 Aqua, via this helpfull link, which luckily in this case was an offical ROM.