Adding new tenant to month to month agreement

[Jill Clark]

Hello!

I purchased a 4 plex over a year ago and inherited three tenants who's residency  ranges from 3 years to 26 years. Besides the occasional whine and whimper, they've been good. 

One unit that includes a mom and adult daughter on a month to month agreement is looking to remove the adult daughter from the agreement and add a new tenant (the mothers boyfriend). Besides the obvious (running all checks on the boyfriend, having the adult daughter give proper written 'notice' and signing a new month to month agreement) I'm curious how to handle the deposit - in our purchase of the property we were transferred the deposits but not provided any detail of which tenant paid what. Does/should the deposit stay with the unit? Is it worth refunding the deposit to both the mother and daughter, then require a new deposit? Is there another and better option that I'm not exploring? 

Any and all suggestions from someone who has seen a similar situation is greatly appreciated.