• The data for units/abiliites, etc. and many useful triggers have been in the SC2 editor since 2015, though many things were bugged and had to be fixed. Other stuff like CGI movies and in-game dialogues I had to import myself by ripping them from WC3.

• The maps themselves have to be converted. The SC2 editor has a tool for converting WC3 maps into SC2, which works, but is fully automated so many things break in the transition, and fixing that is most of the work. Triggers, which are the most important part of campaign maps, don't get converted at all; remaking them takes a lot of time.

• The UI was overhauled. Since only the a few things were available, we remade pretty everything else, which was an incredible amount of work.

So, the WC3 3D models have been in the SC2 editor since 2015, as well pretty much all of the textures and buttons. Map-specific campaign stuff, like voice acting audio files and CGI cinematics, I ripped from WC3 and imported them into SC2.

The editors are quite similar, to the point where you can convert WC3 maps to SC2 using the SC2 editor (though the resulting map has to be fixed in most cases). But in 2015 Blizzard ported all the WC3 models into SC2 (a remarkable feat, considering they use a different format and converting them isn't a straightforward process), as well as all the textures (images that the models use and buttons, for instance) and the data for all the abilities and units, which is an absolutely mindblowing amount of work. They also included HD versions of some models, namely non-campaign heroes and their summons and some miscellaneous units and structures.

So all of this has been in SC2 for 8 years, and yet no one attempted to remake WC3 in SC2 before now (well, there was the Armies of Azeroth project, but they disbanded shortly after Reforged was announced).

Azeroth Reborn is possible because this data already exists, so we can manage with a small-ish team of about 8 mapmakers; otherwise it would be an insane undertaking. Still, it takes 6 months to recreate and polish a campaign, as well as adding new features, so while having the data helps immensely, there's still a ton of work involved to make everything work smoothly.