How to replace / subsitute using index / match?

Formulas cannot replace data, you need VBA for that.

A cell can contain a value or a formula, not both.
So if your cell has a value in it already it cant have a formula as well which would effectively change the value which is what you want.

Formulas can read other cells but not change them which mean you could create a Helper column (a new column) with a formula to produce the value you want.
But you would have to direct any users to use the Helper column NOT the original cell column which would still contain its original value.

Or you can use VBA to change the value of a cell.

??? If you overtyype D4 with 150, K4 updates. if that is not what you want, what DO you wnat????

You'll need to use VBA.
With formulas if you replace original value, the formula will recalculate.
In fact, you'll get circular reference error.