It's not a straight-forward conversion, surprisingly. I'm using Nimble Writer, since it seems to behave the best of the group of tools I've used, but I'm going back through each issue, fixing missing bolds, italics, underlines, replacing single dashes with em-dashes where appropriate, along with fixing any grammatical errors that I may spot.
Once I have it all pulled in to Nimble and formatted, then I can export an ePub, but that's not the whole story.
A cover's needed, which I'll work on as well, along with some manual tweaking of the output CSS to make the resulting book look good on a reader. I've noticed that the exporter in Nimble doesn't export with similar results between multiple formats. To regular HTML looks different than to ePub (which is zipped HTML with images and stuff).
So as a result, when I test, it'll be against my hardware reader, and against the latest version of Calibre, which is where I'll be editing as well (since Sigil is missing a few tools to make my life easier).
At some point, hopefully in the next day or two, I'll be able to ask for a volunteer with a reader that accepts ePub, along with a volunteer with a Kindle, to take a test run through the book and see if formatting is in the pits.
But for now, loves, I am exhausted. Good night! :)