I figured it out.
I finally figured it out. I only replaced the "big" texture, I didn't realize that the objects also had smaller versions of the same texture. I don't know if it works this way for all objects, but for the billboard and watertower at least, I saw there were at least 8 files for each texture, each one half the resolution as the last.
When I went back and resized the original texture I made by half and applied these textures to their appropriate smaller versions, I got it to work at all zoom levels.
I'm still new at doing recolors, so this was a silly newbie question. But I'm jazzed, now my custom 'hood has a water tower with it's name on it!