Yup, thanks for your reply. Unfortunately what you suggest is actually what I hoped to avoid: I have no extra storage for those ROMs other than the SD where they are right now.
Also I made some bootcmd customizations, and other tunings as well, so I hoped to be able not to do everything manually again.
So the only alternatives a far as I have understood are buying a new SD card and moving all the settings or sticking with Rpi3, right?