Release Notes: docs.sophos.com/.../index.html
"Old" V18.5 MR3 Thread: https://community.sophos.com/sophos-xg-firewall/b/blog/posts/sophos-firewall-v18-5-mr3-is-now-available
The V19.0 EAP Community: https://community.sophos.com/sophos-xg-firewall/sfos-v19-early-access-program/
About the Upgrade Issues:
See Known Issue List in Release Note
I have been running V19 on my personal firewall's since the public beta started. Here are my thoughts so far. I also upgraded a cluster of XG230's to test on tomorrow as well.
Upgraded, everything seems to be up and running.
Keep on testing.