Forward doesn't follow traditional releases and doesn't have them.
Instead, starting with snapshot 1, Forward will give people an 'upload when they're ready' cycle with the latest features. It's built for 64-bit processors only and requires UEFI.
It is possible to do a legacy install with Forward, although it'll wipe all data.
Legacy for now doesn't mean anything because it's identical to the technical preview. But future builds will use 'testing' while Legacy is a long-term support (LTS) build and won't receive the latest changes like Forward will.