I did exactly that this morning. I have a disk image backup of my whole system from the day before I upgraded to version 8.
Do this entirely at your own risk, I cannot guarantee it will work for you or that you won’t lose data!
I uninstalled version 8 choosing the option to retain the database (just in case!). I renamed that database folder eM Client 8. I switched off my internet connection. I then installed version 7. I renamed that database folder eM Client 7. I opened my disk image and found the database folder from the day before the upgrade to 8. I copied that folder over to my current location for the database and that was it. When I switched the internet connection back on the emails updated and I am happily back on version 7.
There may be better ways to do it but it worked for me.