FCP7 - how to downgrade from High Sierra

I am in the middle of editing a project for a client, when someone at the office updated my mac to High Sierra,

so now FCP7 will not open anymore.

MacBook Pro (Retina, 15-inch, Mid 2014)

I do not have a Time Machine backup.

So my question is, can I downgrade to Sierra, whilst keeping my FCP7 software intact?

(do I need to reinstall FCP7 btw?)

If so, does anyone have a recipe for downgrading to Sierra, that they know works from experience?