Wasm only gets additive changes - the binary format can't change in a way that breaks any previously existing programs, because that would break the Web. So, you just have to add more opcodes to your implementation.
It introduces new types (structs and arrays), a new section for tags, and several dozen instructions (first-class functions, GC, tail calls, and exception handling). It generalizes to multiple memories and tables, as well as adding 64-bit memories. The binary format changes aren't too bad, but it's a fairly big semantic addition.
