Jenna,
According to:
http://www.scb.se/befovalfard/sakomraden/namn/namnsok.asp There are 1957 persons today in Sweden with Rosenberg as lastname.
The Swedish Telephone Directory on CD-ROM has a list of 1133 persons with that last name, with some spelling variations.
The Rosenberg name is spread in the southern half of Sweden quite well distributed, and that makes me believe that the name was taken on by several families in different parts of the country. I is quite typical that from around 1825 and up until a namelaw 1900 people. There were to many people in the viallages small towns and cities with the patronymicon lastnames (Andersson, Svensson, ie the son of Anders, the son of Sven) and it got confusing, so they started to take new lastnames.
Well, why didn't he keep his lastname Hjert? Possibly because it was not a family name, as you have made an good guess. There was no draft military service in those days, they were contracted soldiers, sometimes for all their lives, but if a soldier Hjert from a certain company would die (in war or by illness) another Andersson or Svensson would have to assume his name.
There is also a wellknown TV-reproter by name Göran Rosenberg, but he is from jewish decendants, probably German or Norhern France, (does anybody else know?).
In the Skarabogs län, there are still some Rosenbergs. Skövde, Borås and Skara are some of the towns in that area with some 10 Rosenberg's
Regards,