Others have posted some good reading on the subject - but you basically abolish the current charter, rewrite it, and have the public vote on it. In the gap between the measure being passed and taking effect the city government gets the new structure in order before the charter switches over - you reinterview everyone, you rehire some portion of the existing force, and you do hiring for new staff all in advance of the switch.

Of course, BPD being BPD, they would flip a tit and possibly burn the city down before that happens, but thats kinda why it has to happen.

Part of it is going to probably require state and federal aid to train up new officers with a new academy and reformed training. This is probably something the city cannot do with just its own money. But the state of MD has been interested in varying regards of doing this for a long time because it hurts the entire state to have the largest economic center in the state have such a corrupt and broken police force. I'm pretty sure the state level could afford the transition, but federal funding might also be involved since its so close to DC.