>>12489232The difficulty lies in remaining compatible with those who do not oppose an upgrade but may still run old clients.
If you make the rules stricter (for example certain inputs are now unspendable) then you're not breaking the old rules, and everyone running an old client will happily accept blocks with the new stricter rules.
Another example are currently undefined witness versions. Undefined versions are seen as anyone-can-spend, and so it doesn't matter what "signature" you add, because you're allowed to spend the coins anyway. Now if you wanted to add a new signature version (for example schnorr based), then most would agree with a new stricter rule that says: this version can only be spent by a valid schnorr signature. This is not problematic if a minority is running an old client, since in their view anyone can spend the coins anyway.
Of course, I have no clue how the qr upgrade will be handled but it will likely be similar and with a long enough deadline.
