[schoen]

Smaller changes have resulted in hard forks in Bitcoin:

https://en.wikipedia.org/wiki/Bitcoin_Cash

https://en.wikipedia.org/wiki/Bitcoin_Gold

So yes, it is conceivable to change the fundamental algorithms, but it would certainly be experienced as a fork to the extent that not everyone involved in Bitcoin participated.

Also, changing the digital signature algorithm is a bigger and more difficult change than the hashing algorithm in some ways, because you need a strategy to preserve people's existing balances, either one that requires, or one that works without, their active involvement (that is, generating signatures). If there is a feasible attack to forge signatures, then a method that can work without existing Bitcoin owners' involvement will leave them vulnerable to having their holdings stolen by forgeries in the future. You can think of this as somewhat akin to weak RNGs in Bitcoin clients (there have been a couple of these) generating vulnerable private keys -- if legitimate owners knew about the problem before attackers acted, they could act to protect themselves, but if not, not. A feasible signature forgery attack based on a quantum computer would put everyone who owns cryptocurrency wallets based on the vulnerable signatures in a position like that of people using a client with a weak RNG. :-(