For my own knowledge is e-pub a brand? A specific app? I am assuming that this conversion is a function of where ever your Word file is being uploaded to. In which case I suspect that eNGINe is correct. There is some setting in that app-web site. Not Word.

Also, I think it may be a very good question regarding styles - that would be the way I would do it. So....are you using explicit styles in yout Word doc?