Mac Pro "Six Core" 3.33 upgrade question

I have a Apple Mac Pro "Six Core" 3.33 (2010/Westmere) currently running Yosemite 10.10 on the internal hard drive. Also have a SSD internal drive which I boot from running the same set up.

I want to update the non SSD boot disk up to High Sierra and understand this will require a firmware update.

Q: If I update the firmware, will I have issues booting from the SSD running Yosemite 10.10 after updating to High Sierra on the other internal drive?

I'm wanting to get this machine up to the latest OS possible but still have the option to 'go back' to 10.10 in-between all this to avoid any downtime.

Thanks!