You can not legally change them without running the risk of getting banned for modded data files.
The idea would be to to use a custom skin locally via modding your game data that the map loads. However this would only be effective to a certain degree as imported heroes will overwrite local ones meaning you would have to modify the map data.
Modifying map data is impossible as there are checksum checks to make sure that the version you are playing is the same as everyone else. Thus you would have to modify it in a lot of ways using cleaver scripts so that the cheksum system still reports the same despite the map being different.
Both might result in you getting banned due to attempting to alter game data or trying to bipass their checks.