changing trainer battle music

hey so i had a question about whether or not it would be possible to change what music plays during a specific battle. for example, if i want to make a regular trainer have the gym leaders theme instead of the regular battle theme. how would i go about doing this? any help would be appreciated (talking about FR of of course)

Are you referring to changing the battle theme to another theme for all trainers, or just specific trianers having a different theme?