Make sure to check the upgrade instructions before doing the upgrade!
Here are some basic stats about the release:
- about 1200 issues fixed in 9.0 (275 were also integrated into 8.1.x branch and released in 8.1.x updates)
- almost 5000 changes were committed into the core codebase between 8.1 and 9.0 release (with another 3000+ changes affecting the bundled plugins)
- 302 days has passed since 8.1 release (almost 10 month, well, let's hope this postmaturity makes the baby stronger :)