Does flash memory ever change on execution?

I'm required to do a code verification using CRC. In this case, all I do is pass every byte found flash memory through an algorithm to calculate the CRC and compare the result to a predefined CRC value.

However, I'm hung up on the idea that the flash memory might change at some point, causing the CRC verification to fail.

Assuming that the code isn't touched again whatsoever, is it possible that flash memory will change during execution? If so, what can cause it to change? And how do I avoid said change?