Regardless of whether it was the right decision or not, how do you move forward? At this point I think you wait until the end of the lease and see where things are at. They may replace the fan and put in the old fixture in whatever condition it is now, or maybe they realize it is damaged and buy a new one to put in. Just follow the lease and the terms you agreed to for the fan and charge for damages as appropriate.
One other thing is you could pre-emptively go back and ask them to keep the fan. This may keep them from doing more electrical work if they agree, but at this point they paid for the fan and there is no agreement for you to keep it, so they don't necessarily have to agree. You may be forfeiting damage to the old fixture, but maybe saving from them doing more work. If that is the route you want to go, but they want to keep the fan, you could always buy them a new one.
Regardless, I think you need to honor the agreements already made. If you want to make new agreements that both sides sign, that may be a great option as well.