Thanks for your comments.  In general, what you wrote makes me feel better about BTC, but the one issue that nags me is indeed the historical transaction records.

I don't understand much about the technicalities of the blockchain, but my impression is that historical transaction records are basically like building blocks upon which all future transaction records depend.  If that's the case, my worry is that if they change something fundamental about Bitcoin protocol, could it potentially create the necessity to go back and rewrite the entire history of transactions from the very beginning?  Using a stupid analogy (but hopefully it serves its purpose), if the transaction history thus far has been build with bricks and then one day the advent of steel comes along, could it necessitate the tearing down of the entire brick structure and complete replacement with steel - which, by fundamental design of Bitcoin, be essentially impossible?