Tenant leaves early, but found replacement

[Robert Fyall]

Hello - 

1 of the 3 room mates is leaving 4 months prior to the lease being up.  She found a replacement who gets along with the other two tenants.  They have been great, house is clean, etc. 

Should I have all 3 sign a new lease?  Do the walk-through again as if they were all moving out and then start fresh?

Are their documents for this type of situation that need to be signed?

They tenant who is moving out wants her security deposit back.  We are willing to do this because she has been good, but are not sure of the protocol here?

Any thoughts?  Thanks!