MBP 2011 A1278 multiple disks with multiple os installs

So... There's a MBP 2011 with an SSD and an HDD (in a caddy), the OS is on the SSD, and I don't know what is on the HDD (sorry, my girlfriend's computer). I suppose that the HDD is dying, she would like to downgrade the OS, because she wants this computer to freeze to 10.10 to use only for Aperture. I think the easiest way to change the drives to a bigger SSD (the one in the mbp is an old 128gb, the HDD is the original 320gb), install the original OSX from apple recovery, and upgrade it to 10.10. What will happen to the "bios"/firmware of the mbp, if I do like this? Can I change back to the original disks, the mbp should work as before? I'm newbie in Apple stuff, I use PC for a long time, sometimes with more than one disks with different OS installations, but I don't know, how does it work in this case.