I have a suggestion how Classic Mode could be done without waisting too much time on extensive menu redesigns.
The main problem seems to be the fact that a lot of menus use buttons with just a symbol on them, which would make them really difficult to understand when those symbols are just random ASCII glyphs.
But what's the actual main reason for Classic Version? Backward Compatibility with old tilesets and mods, so people can keep playing the game like they used to.
But there is really only one place in the game where you can mod in things with tiles: The main map (Creatures and Items).
Also, all old tilesets are really optimized to make either the main map or the world map pretty (this includes the colors.txt file).
So my suggestion is this:
For Classic Mode, just display the main and world maps like they used to be displayed, using the old tileset,
and let an artist design a monochrome, ASCII-like menu look (using the new tile system), that is then colored using the colors.txt colors, so it fits with the rest of the game color wise.
That way no additional work has to be done on the main- and world maps, maybe some slight work to get the menus to work with the colors.txt (I don't know how the code works exactly, obviously), and all the boxes are ticked in terms of having a pretty, backwards-compatible Classic version of Dwarf Fortress, that all the previous tileset users can enjoy, and is hopefully done with not too much programmer & artist time.
Edit: I just heard about TEXT mode. That would of course not work with this plan, but since you can't click buttons in that mode anyways, and it is quite niche, the original plan of replacing all the menu tiles with characters could be used, but only activated in text mode. Since in text mode you can't click buttons anyways, you it seems like a passable trade-off.