It may appear that old soft continue to process data without upgrade.Why you don’t want this? The current situation is that software engineers all like forward and backward compatibility.It is usually win-win if your new application and its old version can operate each other.
Forward compatibility is more rare than backward compatibility. The reason is that if you add some new functions for your software or protocol,the old version will can’t understand it.sometimes you can make the new function as optional,which means it is OK if the old version ignore the new function.This is so-called function degenerating,a popular technology.It is especially useful when a website add a new function but some users want to visit with old version browser.
Forward and backward compatibility is very common in the software engineering,so what is the difference of Bitcoin.The reason is very simple.
Forward compatibility will enable all targets of a node invalid.
It is easy to explain,the downloading of the two versions XT/Core,and their dealing with every blocks all have to begin from the initial blocks.The purpose of the process is to audit the blockchains.
not finished,to be continued……