It looks like some of the documentation related to upgrading to 5.4.2 is not finished yet.
The Upgrade Path for FortiOS doesn't yet list 5.4.2.
The Release Notes say things like: "FortiOS version 5.4.2 officially supports upgrading from version 5.4.0 and 5.2.7." I would hope upgrading from 5.4.1 would work!
The Cooperative Security Fabric has only had its title changed to reference 5.4.2. The 5.4.2 version isn't referenced anywhere else in the document.
I really want to see the Cooperative Security Fabric document and the Upgrade Path both updated before trying this. Important to know, for instance, if a 5.4.2 FGT can work correctly with 5.4.1 FortiClient, FortiAnalyzer, or FortiAP. Also if a 5.4.2 FGT will have any problems with IPSec VPN to a 5.4.1 FGT.
The Upgrade Path now says that you can upgrade from 5.2.7, 5.2.8, 5.2.9, or 5.4.1 directly to 5.4.2. However, it only shows 5.4.0 upgrading to 5.4.1, not 5.4.2, so maybe that's supposed to be a two step upgrade.
The Release Notes don't match the Upgrade Path. The Release Notes now state: "supports upgrading from version 5.4.0 and later and 5.2.8 and later." That implies you can upgrade directly to 5.4.2 from 5.4.0 and can't upgrade directly from 5.2.7 -- both disagreeing with the Upgrade Path.
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.