That is the point: with a soft-fork type of change, old clients would not even be aware that the change was
deployed and activated, unless they read about it somewhere.  How many bitcoin users follow the forums
(and know about the reward schedule)?


And would use the garlic spray and silver bullets in case of a vampire attack...

Switching the PoW algorithm will not jam the miners' equipment.  They would
continue mining as always, and every user or business will continue to
use their blockchain by default, "secured by a network with 1.5 PH/s
of mining power". 

What "switch the algo" actually means means  is that the Faithful will create
a new altcoin, say "TruCoin", that starts off with the current state of the bitcoin
blockchain, but has only a modest amount of CPU/GPU mining; and will ask
everybody to ditch their bitcoins and use only TruCoin. 

But, to  do that, users would have to download a new version of the software. 
How many do you think will bother?  My guess is fewer than 5000 bitcoiners...

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