I don't know whether there is some kind of progress here but a new quantum-resistant public key algorithm could be created and added as a soft fork.
There are actually several approaches, I suppose the most promising ones are error-correcting code or lattice based code. An error-correcting code would additionally need to transmit it's "key", which can't be encrypted by it's own encryption, leaving attack possibilities open. There are already different (already aged) concepts of currencies with lattice based code, but I'm not sure, if it's possible to implement those with a soft fork.
https://arxiv.org/pdf/1203.4740.pdf "Quantum Money from Hidden Subspaces" is a wonderful example of a quantumproof concept