How to write a function that changes values within a matrix a certain amount of times, and saves results in a new matrix.

I have two matrices, x and y, which are each composed of 400 random values all between 1 and 20. I want to randomly select values of x and change them to a new value (also between 1 and 20), and then use an equation to check the similarlity between x (which has changed at 1 position) and y (which remains unchanged throughout). This similarity value is a number that I would like to save in a separate matrix after each time the function is run, and I want each change in x to be carried over to the next iteration of the function until a certain condtion is met.

[Catalytic]

And.... what problem have you encountered?