I think they are using the term "merging" rather loosely. To me merging and replacing are two different things.
As for the England census - they may very well be identical and it may very well be ok to "replace" one with the other.
I'm just saying that Ancestry.com may have had some obscure reason that is not readily evident about why it created two different sources for these items. For example, it may be treating each county as a different database. I wouldn't merge them out of an abundance of caution - but that is just conservative me!