Nothing in your SLA says you have to have servers 1, 2 and 3 available. It says you have to have service X available. You can do that with out taking a downtime hit.
Feel free to correct me then. I have YET to work anywhere in any industry (financial primarily) that stated we had to have specific hardware up and running as part of an SLA.
It's an "SERVICE LEVEL AGREEMENT" not a "SERVER LEVEL AGREEMENT".