Blockchain can be changed... There is two reasons why someone would want to change the blockchain.

First reason is that the original tip caused an error in the daemon that resulted in miners unable to mine more blocks. One common cause is difficulty overhead, which is caused by sudden drop in effective network hash rate that the daemon can't counteract because the next target difficulty would underflow.

Second reason is when there is network split and cumulative difficulty of two or more network segments are identical up to the maximum irreversible length. Because after certain chain length, the daemons refuses to switch to alternate chain, segments continue on their own chain until stopped and rewound.