Hopefully this will help.
TrueOS Legacy is FreeBSD 12-CURRENT plus work from the FreeBSD graphics repo plus Lumina making a user friendly install (think Ubuntu). The Great Desktop was simply Lumina on top of the FreeBSD base.
TrueOS as “a CoreOS” is basically TrueOS Legacy minus the desktop; more of a server install. This fits in nicely with iXsystems NAS products. Think of it like HardenedBSD: a fork of 12-CURRENT, so they get better hardware support and can integrate fixes quicker. I imagine any fixes will get pushed back upstream.
Trident is basically a user friendly install of desktop environments on top of TrueOS (as a CoreOS), so you wind up with what you have now (TrueOS Legacy).
The biggest difference I think between TrueOS (as CoreOS) and Trident is how packages are built. A server typically wants different options than a desktop.
If you do nothing but wait, I believe that at some point your current TrueOS Legacy (stable and unstable) will tell you “updates available”, you update and when you reboot you are running Trident.
Trident coming up to speed I think is mostly getting the build process worked out.
Above is my opinions, which may or may not reflect the reality of what is going on.