Seriously, this is so tiny an issue that publishers who "do" a British edition and a US edition have custom macros written to do the spelling and punctuation changes.
That's what my US editor told me. HA! My galleys on book one were so messed up it was silly. Took me about five extra hours just looking for the spelling and punctuation changes they missed.
So for book two (which was already written at that point), I ran my own series of search-and-replaces in Word to fix it. Took less than one-half hour. (Okay, if I were actually a techie person like, say, Medievalist, I would've written the macos myself. But I ain't.
)
For books three and four, I wrote in American English from the outset.
It helps that I'm American so know American English. If I weren't, I'd let the publisher do it.
Hey! Does this mean I'm not as sadly monolingual as I claim to be? I know both British and American English! Maybe I'm bilingual, after all. (With apologies to those who really ARE bilingual.)
(Oh, and yes, I know that the major publishers really do have those macros/scripts. But if my publisher had them, the production folks apparently didn't know. Sigh.)