Blockchain security and SHA256

While my question is highly theoretical obviously... if a single person were to find a way to break (or solve) SHA256 and discover the nonce without having to brute force, how easy would it be for them to rewrite a large portion (hopefully not all) of the blockchain since now they can write and submit valid blocks very fast creating longer mathematically valid chains than anyone else?

(Could they just create a bunch of private wallets, and make a large number of transactions between them and just keep mining and collecting the rewards on those?)