This blog post discusses F/A-18 Hornet 2.0 and whether it's a sequel or an upgrade to Graphism's first Hornet game, F/A-18 Hornet. It is based on what I learned as I worked to get the game running under modern hardware with emulation so it may not be 100% accurate. If you can expand on or correct anything, please feel free to get in touch.
Before we begin, it's worth differentiating between the game title, F/A-18 Hornet 2.0 and the version, v2.0.1. This version seems to be the release version distributed on cd. I haven't seen an earlier v2.0.0 version.
The short answer to this question is that it is both, an upgrade for some of Graphism's customers and a sequel to others.
Upgrade path
For customers that purchased one of the earlier versions of F/A-18 Hornet, it's an upgrade. An upgrade package came on 4 disks. A quick discussion of the myriad of  F/A-18 Hornet versions can be found in the blog post: F/A-18 Hornet: History of a classic.
The upgrade application would upgrade any v1 copy of F/A-18 Hornet (presumably v1.0.0 through to v1.1.3b2, but not Classic) to F/A-18 Hornet 2.0 v2.0.1.
Installation path
Those customers without a copy of F/A-18 Hornet, or those that purchased F/A-18 Hornet (Classic) would have to perform a fresh install of F/A-18 Hornet 2.0. The game was distributed on a single cd. The upgrade application is incompatible with the Classic version, and for those customers it would be considered a sequel.
Korean Crisis, expansion or upgrade?
The Korean Crisis addon could also be considered both an expansion and an upgrade. For customers with an earlier version of F/A-18 Hornet (v1.0.0 - v1.1.3b2), the installation would upgrade to F/A-18 Hornet 2.0 v2.0.1 with the Korea theater added.
For those customers with an existing install of F/A-18 Hornet 2.0, Korean Crisis would only add the Korea theater, the version remaining unchanged at v2.0.1.
Contentious?
The upgrade and expansion paths probably led to quite different and possibly contentious customer experiences. Both F/A-18 Hornet (Classic) and F/A-18 Hornet 2.0 were released the same year, 1995.
A PowerPC customer, may have purchased F/A-18 Hornet (Classic), to get an optimised PPC version and then had to purchase F/A-18 Hornet 2.0 and Korean Crisis, to get the definitive F/A-18 Hornet 2.0 experience. Whilst another customer may only have had to buy F/A-18 Hornet v1.x followed by Korean Crisis to get the exact same definitive edition.
It's my subjective opinion but the F/A-18 Hornet (Classic) version seems to have been a mis-step by GraphSim, that created confusion. It may have been better to have released a PowerPC optimised patch (the PPC Hack patch shows this was feasible) rather than a new version, based on v1.1.2 and incompatible with v1.1.3b patches or upgrades to Hornet 2.0.