You could also install a motion sensor switch. I feel installing a dimmer wouldn't accomplish the goal of saving utility costs. The tenant could leave the dimmer turned all the way up and just turn it on and off all the time just as they would with a regular switch. The motion sensor would at least make sure the lights turned off when no one was around.
I believe the idea behind this is the dimmer switch is to reduce your expenses, therefore increasing your NOI, which raises the value of the building. The year you buy all new switches and install energy efficient products your expenses may be higher but the following year your accounting should reflect a higher NOI based on you saving more on energy costs. I would consider motion sensor or timed switches in public areas and replace all bulbs with LED. leave the regular switches the way they are. The more units the more exponential the cost reduction could be. I would do my research and really see if this could pay off. Also to receive the benefit of increasing the value you will need to do a cash out refi or sell the building.
Not sure if you listen to the pod cast, but in Ken Mcelroys podcast (not sure on what number) he talks a little bit about how to add value to multi families. Ben Leybovich also talks about valuing multis and how to add value in his pod cast.