RAM and Flash structure difference
if I would replace a NOR Flash in a system with a NAND Flash that contains the same data


Trouble is that you usually cannot get a NAND and a NOR flash with the same data interface or protocol. In other words, the existing system won't know how to access the new chip properly.

And you really don't want to talk to raw flash directly, as e.g. SD cards are actually cheaper than (tested) raw flash chips that store the same amount of data.


  when NOR Flash should be used and when NAND Flash should be used


Depends highly on your application - how much data you need and how often you will write to the flash, and which granularity you require (erase block size).  Oh, and which interface you'd want to use in order to talk to the flash chip.