You could replace ECDSA with a post quantum algorithm. Keep in mind that many crypto primitives are safe, so there are large parts of bitcoin where you don't have to do anything. Digital signatures is going to be where the main problem is for bitcoin. But thinks like the hash algorithm should be fine (at most quantum gives a square root speed up for hashing, which isn't enough to be really concerning).

One thing that might be problematic for a blockchain where everything has to go on the blockchain forever is that some post quantum schemes have really large signatures or key sizes.

I'm not that familiar with the details of bitcoin, but i had the impression that p2pkh is more secure against quantum computers.

[I should emphasize, im not a cryptographer and only somewhat familiar with bitcoin]