I have just finished doing my first epub makeover that I downloaded from Gutenberg's.
I used calibre to convert it from epub to a .txt file. Opened the txt file in MS Word 2007, did my editing, saving my changes in the books file in Calibre.
The only changes I really made was the stuff that didn't scan properly, some spelling errors, but nothing to change the authors style.
I than converted it back into epub, using calibre, and voila a book that is easier to read!