Are the characters British? (Or one of them?) Is it set in England? (Or Australia?) If so, leave it. If not, why not go ahead and make the changes? You know your market is "American" (technically not since HQ is based in Canada, but we both know what we mean. <G>) and you know which words need to be changed, go ahead.
I don't think, however, that it will impact the sale of your ms. The editor won't say, "This is a really good ms and fits our line well, but gosh, I'm just not going to buy it because the silly author put some British spellings in there."
I repeat, an editor will NOT reject a book that's otherwise buyable just because of some spelling differences.
So, take your pick. Leave them, or change them, according to what feels right to YOUR gut.
Here's an interesting side-note on spellings. My second book was called The Mommy Plan. When it got released in Australia, and the UK, the title was changed to: The Mummy Plan. <G> Didn't matter that my characters were Americans, and the story was set in the US. They changed it. Hey, I'm all about marketing, so it didn't bother me at all.
Even more interesting, however, is that they didn't change the word in the TEXT of the story. Just in the title.
Susan G.
