Few years ago Ubisoft released the HD version of Heroes 3 on Steam. This edition includes nice HD assets that make the game look great on modern resolutions. The major downside of Heroes 3 HD Edition is that it lacks both expansion packs which means that the players are limited to Restoration of Erathia content only. However it's now possible to play Armageddon's Blade and Shadow of Death using the HD assets. The Heroes 3 fan sourceport called VCMI introduced a few days ago support for HD assets from the Heroes 3 HD Edition available on Steam, although keep in mind that you need to own both the Heroes 3 Complete (https://www.gog.com/en/game/heroes_of_might_and_magic_3_complete_edition) and Heroes 3 HD Edition (https://store.steampowered.com/app/297000/Heroes_of_Might__Magic_III__HD_Edition/). Here are all of the steps that you need to do to install everything:

1. Install Heroes 3 Complete from GOG and Heroes 3 HD Edition from Steam.
2. Download and install the VCMI sourceport (https://vcmi.eu/download/).
3. On the first launch the VCMI will want to import the needed files from Heroes 3 Complete. After that the game should work without any problems through VCMI with standard assets.
4. Download the HD asset converter (https://github.com/Laserlicht/vcmi_hd_mod) and run the .exe file.
5. The app will ask you to point the installation folder of Heroes 3 HD Edition from Steam and the target folder where the converted HD assets should be stored (it can be any folder).
6. When the conversion process is complete you will get a notification, although keep in mind that it can take even one hour (it depends on the hardware).
7. Next put the "hd_version" folder with converted assets to the VCMI mod folder. It's default location is "Documents\My Games\vcmi\Mods".
8. Activate HD assets in the mods tab of the VCMI launcher.
9. Enjoy Heroes 3 and its expansions with HD assets. :)

Keep in mind that the assets introduced by AB and SoD expansion packs will be still in SD, but it might be only a matter of time when some modder prepares them in higher resolution. Also VCMI introduces some changes to Heroes 3 functions and has a custom-built AI (with optional module that further increases the difficulty level) so don't be surprised if you encounter some differences in comparison to standard version of the game. Most of the changes are only quality of life improvements though, similar to those introduced by popular HD Mod. VCMI gives however almost unlimited possibilities. Right now you can install around 20 additional types of towns. There are also mods introducing new artifacts, heroes, town screens etc. You can also install Horn of the Abyss and Wake of Gods unofficial expansions although it's not suggested to use them yet with HD assets because they have a tendency to replace them with SD versions. There's also support for Heroes Chronicles if you want to. :) VCMI works also on Android and iOS devices, so you can now play Heroes 3 everywhere!

Ugly low-quality upscales, though. Because original source art is lost and Ubies failed to do a proper remaster as a result. So, it's good to have this possible technically but completely pointless.
One more nail to the coffin of the idea you mentioned yourself: only RoA is upscaled. While everything more or less modern fan-made requires at least SoD. And you'll get a patchwork of Ubies' ugly upscale and machine upscale of the other graphics resources.

Actually the assets from the HD version are quite good IMHO. What do you expect from a remaster? It's not a remake. The HD assets were not simply upscaled by Ubisoft but remade keeping all of the characteristics of originals. Even today AI isn't capable of upscaling textures that good and there were no such tools a few years ago. You can check what effect gives a simple upscaling by activating xBRZ algorithm in the HD mod or by using Lossless Scaling app. It's impossible to achieve with these tools a similar effect to the HD assets from the Heroes 3 remaster. Thus the only meaningful problems with it for me are the lack of expansion packs and QoL improvements from HD mod. VCMI roughly solves both. I played today with Conflux town and it wasn't that bad. You can definitely get used to the situation when some assets are still in SD. I'm almost certain though that sooner or later someone will provide HD assets for AB and SoD.

I think this is a great initiative, I will definitely give it a try!

This is great initiative for sure. Still must admit the fact, Ultimate HD never was ultimate and hd been indeed poor quality upscaled.

On custom campaigns I get the following error message:
vcmi unsopported map format. Format ID 32.

I tried Succession Wars campaign.

Is that a known issue?

You should report that on the VCMI Discord channel. Someone should know what to do with this. I don't have enough knowledge about this engine to help unfortunately.

wow this is great news.. hopefully modders will eventually upscale the AB/SoD assets to match the Ubisoft RoE assets and we'll have the complete remastered original campaigns experience.

Succession Wars is Heroes 2.