There is a lot of truth to the advertising ploy - first formulas can't be advertised so this is how the companies get around it.
The follow on formulas are slightly different and do have more iron in them, many babies get constipated on the increased iron and they can find it harder to digest as it's designed for them to have the same nutrients in a smaller volume. If your LO is doing well on the first stage formula there isn't a necessity to change, it's fine for the whole first year!
HTH!
Laura