For a minute I was confused by this elevation, until I remembered that in Wurtemburg they were "only" serene highnesses. Highness with no modifier is higher up than SH, but one step down from royal highness.
Oh those Germans, with their umpteen levels of titles!
There is Grand Ducal Highness in between.
2) Ihre/Seine Erlaucht (Illustrious Highness) f.e. for Counts like Waldburg-Zeil family.
3)Ihre/Seine Durchlaucht (Serene Highness) f.e. Thurn und Taxis
4)Ihre/Seine Hoheit (Highness) f.e. some Dukes f.e. Duke of Saxe-Altenburg.
5) Großherzogliche Hoheit (Grandducal Highness)
I think it's not used anymore. It was used in Lux, Hesse and Baden for the younger children of a Grand Duke. F.e Alexandra Feodorovna was a Grand Ducal Highness Prior to her marriage
6) Königliche Hoheit/Royal Highness
7) Kaiserliche Hoheit/Imperial Highness