Thanks Bonnierat!
One thought occurs to me. Some scenarios seem to modify the graphics. Any deas how I can avoid losing that if I substitute the new file(s). Apologies in advance if the answer is obvious.
Scenarios that modify graphic files have their own subfolders (inside the graphics or AltGraphics folder). I've modified the files on subfolders that come with Toaw, mantaining those that are functional.
Just place any scenario subfolder in your Altgraphics folder, open the files with prefixes 'tiles' or 's_tiles' ('units' for counters) and, if they have only aesthetical value, you may replace them with the corresponding file (same name file) you want.
Make a backup of your AltGraphics folder and just play around. There is no risk, since the original Graphics are on the Graphics folder and any files you place on the AltGraphics folder will just override it when the game is loaded until you remove them.
I have more than one separated AltGraphics folder with different files configurations, so that all I have to do is to replace them when I want. After adding or deleting any file on your AltGraphics folder, just zoom in or out in Toaw and graphics will be updated. If you keep multiple AltGraphics folders like me, it's just a question of dragging them before playing in order to have a different visual experience.