AHA! Now the actual problem is clear.

importedslit0data /. {x_, y_} -> {(x - 2.3585)*74.8, (y - 0.0246623)^2}

does, as you see, transform the data.

BUT that does not then store the result back into importedslit0data, it just transforms it and displays the result and nothing more.

What you have is sort of like you say z=2 and then you say z^2 and Mathematica prints 4 and then you say

Plot z and mathematica plots 2, because squaring didn't also mean to store the new 4 back into z.

So try this.

importedslit0data = importedslit0data /. {x_, y_} -> {(x - 2.3585)*74.8, (y - 0.0246623)^2}

ListPlot[importedslit0data]

That tansforms it, stores the result back into the original variable, losing the original numbers now, and then should plot the new numbers.

So much easier and quicker to get to a solution when we can actually see "all" the code.

Thank you for that

In your defense, this is a relatively common trap for new users to fall into. Again and again I have seen others read the documentation and seem to come to exactly the same conclusion that I think you did, that when the documentation says "it replaces what matches the pattern" that it is assumed it assigns the new value to the existing variable.