Lease is up and 1/3 Tenants moving out. New Lease OR Addendum?

[Rachel O.]

Hi! First time posting here. 

My "joint and several liability" tenants - 3 college boys - lease is up end of May in San Diego, CA.  They have been great - paid on time and good tenants.  Their moms have all co-signed and they have been tenants for 2 years. 

Two are staying and one is leaving. The two staying have chosen a new roommate that I have approved and who has a co-signer as well.  

Since the lease will be up and it has already been extended previously -- is it better to do a lease extension and roommate addendum or just start over with a new lease?  Pro's and Con's? Is there something I am not considering?

The current lease states that they won't get their security deposit back until all 3 terminate their lease.  The new tenant is going to pay out the tenant who is leaving.  Is there a form for this so I can track and not have issues at a later date? 

Any verbiage or paperwork suggestions would be helpful. 

Seems like there must be a very simple way to do this.  

Thank you!