Princess Ena of Battenberg was not elevated to Royal Highness from Serene Highness, but from Highness. Queen Victoria decreed for children of Prince Henry of Battenberg and Princess Beatrice that: “the Sons and Daughters born of the said Marriage of the said Prince and Princess shall at all times hold and enjoy the Style Title and Attribute of Highness" as “being British born Subjects and Descendants of Our Royal House”.
Ena was never a Serene Highness.
Elevating the qualification of a spouse before marriage was not unique to that case: two generations later, Princess Ena of Battenberg was also elevated from a Serene Highness to a Royal Highness, to be suitable for marriage to King Alfonso XIII of Spain. Her uncle, who by then was King Edward VII of Great Britain, was the one who conferred that style to her ...
It's somewhat odd to elevate someone who wasn't born to a particular qualifying rank before marriage. It's one thing for a princess born, say, a Grand Ducal Highness to Imperial Highness -- as happened commonly when German princesses married into the house of Romanov in Russia. But for marriage to an enthroned king, Ena would not have been elevated during her maiden days: she would simply have adopted the style of her husband at marriage, whoever it was. Meaning: if she married a *mere* Royal Highness, then she simply would have become one after marrying -- not before.
If I understand correctly, Emperor Franz Joseph of Austria elevated the entire family of Saxe-Coburg and Gotha to Royal Highnesses, because his daughter-in-law, born Princess Stephanie of Belgium (and member of the said house), was a sister of Princess Louise, who married Prince Ferdinand-Philippe of Saxe-Coburg and Gotha (Kohary).
That had to suite his mother, Princess Clémentine of Orléans (a born Royal Highness), a daughter of King Louis-Philippe of the French.
Elsewhere, I believe that members of the ducal branch of the House of Wittelsbach (to which Empress Elisabeth was born) were originally only Serene Highnesses. They later got elevated to Royal Highnesses sometime during the reign of King Ludwig I of Bavaria.
According to the Wikipedia article, he was given the HRH style shortly before the wedding, on Feb 6th, probably so that Victoria would not marry a "mere" Serene Highness .