Hi Gavo,
I just upgraded 3.3.27 to 2018.8.8 and that went pretty smoothly.
The docs say you can upgrade 3.x to 2018.8.8, and so far i have to believe them 
But just in case, just make sure to have backups available. If you can snapshot the EC2 VM, i think thats easier/safer then doing an application downgrade.
You can read about my experience here: Notes and findings upgrading 3.3.27 to 2018.8.8