JIT and SMC are two different things because JIT is write-once-then-execute, SMC is write-many-then-execute-many. Even with reoptimisation like the JVM, you're not modifying the existing code but writing it onto a new page. That is the key difference, you're not modifying existing written-out instructions, you're creating new ones.

Consider a kernel that boots on several different machines with different capabilities. Patching the call instructions at init time, to point to either one version or another of a frequently used function, saves time.