Bitcoin & crypto
Soft fork
A backwards-compatible consensus change where upgraded nodes enforce stricter rules and old nodes still accept the chain. A post-quantum signature scheme would most plausibly reach Bitcoin as a soft fork.
Related terms
BIP-360
A Bitcoin Improvement Proposal defining a quantum-resistant output type, sometimes called Pay to Quantum Resistant Hash. It is a proposal under discussion, not an activated consensus rule.
Cryptographic agility
The ability to change cryptographic algorithms, parameters or libraries without redesigning the systems that depend on them. Agility is what turns the next migration from a rewrite into a configuration change.