According to Merriam-Webster (American), Macmillan British and American, dictionary.com (American), the Oxford English Dictionary (access necessary e.g. through a university network), Longman (British), and Cambridge Dictionaries Online (British), marshal seems to be the variant more commonly used for the official/military title (in both British and American usage), hence I say the badge name should be changed. As mentioned before, the double-L variant mostly occurs as a last name. Some dictionaries list it as that, others don't at all.
Wikipedia may or may not be trusted in this matter, while http://en.wikipedia.org/wiki/Marshal claims
Marshal (also spelled marshall, more commonly in British English than American English)
Marshall may refer to:
- "Marshall", an American spelling for the military rank of marshal