I pretty much always change cover. Even when I'm very lazy, when I'm not even sure that I'll read the book, I'll do something like this sample. I'll find an inner title (or whatever this is called) somewhere online, and I'll quickly edit it to target readers resolution.
Sometime I'll replace one cover with another, sometime I'll find a photo of first edition cover on an auction site, and use that, but most of the time I'll just make my own using my photos as background, and title and author in fonts of my choosing. My readers make a nice display piece like that, even though they look tacky, I suppose.
Other than that, I'll sometime experiment with initials and first words of chapters, and change chapter size and title. But all of that really rarely. I did fool around with embedding fonts a lot, before readers started to support custom fonts. Lately, I've also started to rip archive.org books into pdf's. A somewhat lengthy process of questionable legality. They have books there that are available to be borrowed, but you can't download source files, or pdf's... So I really don't know, but I guess that I'll keep doing it.
