Superb instructions. Thanks!

My wallet didn't quite follow the script but things turned out fine (I think).

The script said:

step 6) launch the wallet, choose "YES" I want to rebuild the block index

12.1 uses a different indexing format so must walk the entire blockchain.
step 7) take a break, grab a sandwich, enjoy some sun

this can take anywhere from 3 to 4 hours depending on your machine
once complete, enjoy your new upgrades!

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The button actually says "OK" not "YES" (hair-splitting), and in my case the wallet crashed almost immediately after that with one of those unhelpful Windows messages "an error has occurred" (and we know what it was but we won't tell you).       

I restarted the wallet and it seemed to sync up fine on its own in only about half an hour! Balance and transactions look normal.

Very nice!