Special character hiccups – global replace??

Upgrading to 2.6 from 2.5.1 seems to have gone smoothly for the most part. However, occasional characters have been converted to “?”. Some of these were undoubtedly the result of careless copy/paste from non-ASCII text (some ‘smart quotes’), but not all (as far as I can see, some ordinary spaces are also hiccuping); in any case, they were all properly invisible in 2.5.1.

Is there a way of globally replacing the bad characters? There are only a couple per post, but with 150 posts it’s not practical to manually repair this.