The aspect ratio is displayed incorrectly.

Settings and saves for the original game and the add-ons are stored in different locations. For example, for Tribes of the East, they are located at \User Documents\My Games\Heroes of Might and Magic V — Tribes of the East\saves.
The settings file is also located there: user.cfg for the original game, and user_a1, user_a2 for the add-ons.
However, there's no point in these manipulations, as the proportions in widescreen mode will be distorted everywhere except for Tribes of the East with patch 3.1. No matter how you adjust the settings, they are stored within the executable file.
There is an option to play the old campaigns using Tribes of the East 3.1, but you will need to download a mod to unlock the previous campaigns and play without distortions.