There will likely be different character lengths for account addresses and keys, so all of the DNS+HTTP web services and HTTP web forms built on top will need different form validation.
Vitalik Buterin presented on this subject a few years ago. Doubling key sizes may or may not be sufficient to limit the risk of quantum attacks on elliptical curve encryption algorithms employed by Bitcoin and many other DLTs.
The Chromium browser now supports the ML-KEM (Kyber) PQ cipher.
Very few web servers have PQ ciphers enabled. It is as simple as changing a text configuration file to specify a different cipher on the webserver, once the ciphers are tested by time and money.
There are patched versions of OpenSSH server, for example, but PQ support is not yet merged in core there yet either.
There are PQ ciphers and there are PQ cryptographic hashes.
There are already PQ-resistant blockchains.
Should Bitcoin hard fork to double key sizes or to implement a PQ cipher and hash?
Spelunking for Bitcoin by generating all possible keys and checking their account balances is not prevented by PQ algorithms.
Banking and Finance and Critical Infrastructure also need to upgrade to PQ ciphers. Like mining rigs, it is unlikely that existing devices can be upgraded with PQ software; we will need to buy new devices and recycle existing non-PQ devices.
If banks are on a 5 year IT refresh cycle, that means they need to be planning to upgrade everything to PQ 5 years or more before a QC quantum computer of a sufficient number of error-corrected qubits is online for adversaries that steal cryptoassets from people on the internet.