[Mrs_Often]

Are you sure the "text-align" in the CSS only occurs as centred? As you describe it, it seems that there should be another tag for text-align elsewhere for a different style.

In any case, you can convert the book in Calibre as you described, and include "text-align" in the Filter style information to be removed field (see screen shot). Beware though, this will also delete text-align settings for style classes where text is centred on purpose (for titles for instance, or poems, or letters sent in the story).

If that doesn't work the alignment will probably be hardcoded in the html of the book instead of added in the stylesheet file. You could manually Find & Replace "text-align" in Sigil. Once you've found the exact "phrase" used for alignment in that book, you can Find & Replace that and leave the "Replace" field empty. Note that "text-align" is the CSS tag, and in the book they might have used the html "align" tag, so search for that too.

Good luck!