Getting my BTC full node caught up. Doing Dec 28, and the blockchain is up to 606G. At this rate, moving to an external 1TB drive will only last another year.

I noticed on bitcoin-core an option to keep only N Gigs of blockchain history. How does this work? What if somebody sends you some BTC in a transaction that spends from a transaction N+1 or more Gigs ago?

Another question. What is to prevent someone from altering an old block with low difficulty to change a transaction but have the same hash? You could then field a bunch of nodes to return it instead of the original. This is easier when everyone is only keeping the last N Gigs of blockchain history.

This would allow redirecting an ancient transfer to someones cold wallet to go to your own instead.