New lease vs. "switch roommates" document

[Kenneth LaVoie]

I've always had one document for removing a tenant from a lease (in the case of roommates where one stays and one goes), and another for adding a new tenant, if that were the case. Instead I'm considering just doing a "new lease" to make things cleaner. The only thing that worries me is the security deposit. When roommates move in, we make it a big deal to educate them on the "security deposit goes with the apartment, not the tenant" bit. Meaning if one tenant moves out, we do NO under any circumstances refund their "share" of the security deposit. They work that out amongst themselves. This is pretty clear and simple with my old way of adding / subtracting but with signing a new lease, am I somehow setting myself up to wipe the slate clean and refund one party leaving?