Sometime in the next several years, Bitcoin will switch to quantum-resistant alternatives of SHA-256, RIPMD-160, and ECC. Then you simply have to move your bitcoins from the old addresses to the new addresses. Moved coins will be ok, but the rest (including lost coins) will be vulnerable.
Those lost coins will get moved to new addresses which are quantum-resistant. So ... only the coins from genesis block will remain unspendable.
The coins that are not moved within a specified limit of time should be sent back into the mainstream. Their owners probably have lost the keys.
seriously?
how would you feel if your lovely shinies stored for future use suddenly decided to change their allegiance without any reason
just because someone somewhere thought it might be good to set the "lost" coins free?
I'd be pissed
As soon as you find out that Bitcoin has a new quantum-resistant alternative of whatever cryptographic primitives we are using, consider sending your existing coins to new addresses, before someone else does.
Just keep up to date on core development, you'd only have to look maybe once a month if you're too busy to look every couple of weeks. And even if you don't look at your wallet for a couple of years, your coins in cold storage that were never spent would probably be safe.