If that happens I’m pretty sure we have much larger problems than someone stealing your bitcoins
Quantum computers have something similar to moore's law which is that adding each bit is as difficult as all the previous bits. i.e. the required noise floor scales exponentially, unless a certain threshold is reached.
If BTC has a multi-trillion-dollar market cap, and quantum exploitation occurs, the only thing that backs the currency (trust) vanishes, and the values quickly drops to zero.
If we are reliant on BTC for global finances in the future, as many are proposing, this could result in a global depression.
This might be the single biggest problem quantum computing could bring us, short of the "talking to alternate universes" theoretical use cases some people are talking about (that probably won't happen).
Encrypted data can be re-encrypted to be quantum-secure. The vulnerability is temporary
A compromised blockchain dies forever. A brand new quantum-secure blockchain would need to be invented.
More likely: "We'll skim from dormant wallets that won't be immediately noticed, and when it becomes too obvious, we move all of Satoshi's bitcoins to force an emergency fork that launders our profits".
I could exploit the old blockchain, then migrate my stolen keys to the quantum blockchain.
Forking doesn't work here. Traditional computing and Quantum computing are different enough that "porting" isn't just something you can do.
It's a question of how much time a new algorithm buys.