The important thing that nobody has mentioned yet is chain length. A bigger cassette will need a new, longer chain.
It's important to have a chain that's long enough to safely cover your big/big combination. If your chain is too short, and you ever accidentally shift into that combination, it can cause a BUNCH of damage in less time than it took me to type this sentence.
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My greatest fear is all of my kids standing around my coffin and talking about "how sensible" dad was.