Syncing on top of old chain ... works:
...
So ... looks like updating AFTER a new version has already passed the fork-block ... is unproblematic.
config="{ChainID: 1337 Homestead: 0 EIP150: 0 EIP155: 0 EIP158: 0 Engine: ethash}"
Please make a full resync to load the new config. By default fast sync is enabled - so it will take you only a couple of minutes.
config="{ChainID: 42 Homestead: 2000000 EIP150: 2463000 EIP155: 2675000 EIP158: 2675000 Engine: ethash}"
Why is that necessary?
The first 2000000 blocks were correct in any client version, were they not?
Only at 20000001 your new code started to do stuff differently - right?
Because of the ChainID. If you don't plan to do any transactions from this client - then it should work without a full recync, but you might stuck again on the upcoming forks (EIP150: 2463000 EIP155: 2675000 EIP158:2675000).