The reason for my asking is that Ernst II (first cousin and brother-in-law of Queen Victoria of Great Britain) succeeded as the reigning duke in 1844, the year when there was a change in the qualifying rank. But because he was the sovereign, he obviously was entitled to something higher than just plain Ducal Serene Highness. Wikipedia (which once again has to be taken with a grain of salt) states that he assumed the rank of Highness.
What was his father (Duke Ernst I) before him? Was he also a Highness, by virtue of being the sovereign? The reason for my asking is that in some houses -- namely, princely dynasties -- both the reigning monarchs and cadet members are styled the same: Serene Highness. We see this in the principalities of Liechtenstein and Monaco. The same applies to mediatized houses, where the Fürst in each (e.g. Thurn and Taxis) is styled as Serene Highness, just like the other members.
Of course, as Ernst II married but died childless, succession to the duchy of Saxe-Coburg and Gotha eventually devolved upon the male issue of Victoria and Albert (the younger brother who long pre-deceased him, having never renounced his rights to the throne). Because of their status as British princes, the nephew (Alfred, Duke of Edinburgh) and great-nephew (Charles Edward, Duke of Albany) who in turn inherited the German throne were obviously both styled as Royal Highnesses.
However, the children of Charles, unlike the children of Alfred, were only Highnesses. They were only great-grandchildren of Queen Victoria, anyway, and were fortunate that this qualifying rank was at least better than Ducal Serene Highness. Evidently their father got demoted to Highness, after World War I and the abolition of the German monarchies, whereupon the duchy got deposed.
As for his elder daughter, Princess Sybilla: she got elevated to the qualifying rank of Royal Highness, when in 1932 she married the Hereditary Prince of Sweden.
Like I said, the issue of titles and styles is extremely confusing with the Coburgs, since they branched out and assumed a number of thrones elsewhere.