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So lets say I can create SHA-256 collisions fairly easily, and I want to replace an old transaction somewhere in the block chain.
I create an alternate version of the transaction with the same hash... and then? Whenever clients happen to connect to my node to get old transactions I feed them the bogus version?
How do I get a majority of the network to accept the bogus version as valid, when the majority of the network probably already has already downloaded the old, valid version?
Same question if I'm creating duplicate (old) block hashes instead of duplicate transaction hashes.
I suppose I could try to double-spend with two transactions that hash to the same value... and hope that the merchant's bitcoin accepts Transaction Version 1 while the majority of the rest of the network accepts Transaction Version 2 (where I pay myself). But if SHA-256 ever gets close to being broken I'm sure bitcoin will be upgraded so new clients only accept upgraded hashes for new blocks/transactions.