Data validation using indirect

I've attached a sample file of what I am trying to do.

In row 2 I have data validation in the form of lists. Below row 2 is the data.

Cell B2 has an indirect expression referring to cell A2. In other words, B2 is dependent on A2 (cascading)

What I would like to happen is when a new selection is made in cell A2, B2 populates with "Choose platform", instead of keeping the old information. The drop down populates with the new data but cell B2 still displays the old data.

Book2.xlsx