It hasn't changed. Maryland derives the duty to retreat from the common law. But the common law has also held since at least the early 17th century that a man's home is his castle (i.e. his place of safest refuge -- meaning there's nowhere else he can retreat to) and that there is therefore no duty to retreat before a use of deadly force which would otherwise be justifiable. This was established explicitly by the Court of Appeals in 1963 (Crawford v. State, 231 Md. 354, 361, 190 A.2d 538, 541 (1963)) but was the law in Maryland before that date because Maryland inherited the common law from its colonial government.