I believe that L.A. prohibits smoking in apartment buildings as it is.
Check with the City or the RSO board. If so, then yes you can change it and probably are required to ban smoking by law.
Even if not, you can prohibit smoking in your building. I don't agree that you have to get the tenants' approval, especially since they are month to month. They do not have a lease. You can put that in a new lease, or just send out a notice that smoking is not going to be allowed starting on a date at least 30 days from the date of the notice.
Under the RSO, whether or not there is a written lease, the landlord cannot change the terms. without the tenants consent. Here, the tenants have been smoking for 3 years, so it would appear to be a prohibited new term.
As a landlord, I am not aware of any Los Angeles Municipal Ordinance prohibiting tenants from smoking in their own residential units, as opposed to a contractual lease term. If you find such provision as suggested by one the posted, please let me know about it.