Why is it impossible to replace the data in blockchain? Why can nobody spend the same bitcoin two times?
Theoretically this can be done. But resources are the issue. To do this, you need huge computation power.
The data are stored in blockchain blocks. The blocks are not independent. The blocks are numbered and each block contains information (“hash”) derived from the previous block.
Let’s take a chain of blocks, A-B-C-D-E..
If you change data in B-block, hash in C-block does not correspondent to the hash of the changed block. So you canot change only B-block, you also have to change all subsequent blocks.
It will take an extremely high computation power.
Warped jigsaw is an analogy.
Any its part (“block”) is unique. “Miners” create the next jigsaw fragment so its boundary correspondents to the boundary of the previous fragment.
If somebody changes one fragment he has to change all subsequent fragments to save cohesiveness of the jigsaw.