change cell value in same cell
 

I guess I need a macro that will change an existing value in a cell. For
instance, in cell A1, I type in a value of 2. This value has units of feet.
In cell D1, I have a drop down list of "US, Metric". When I choose "Metric",
the units for cell A1 are now in meters. I need a macro, that when and only
when the I chose "Metric" from D1, it takes the existing value in cell A1,
and then runs an equation on that value and displays the new value in A1. I
then, can change the value at will, without the macro running the equation
each time I change the value in A1. Then, I would need this macro to also
due the reverse, if I then choose "US", it needs to take the existing value
in A1 and run an equation to take the existing value and change it to the
correct US value. Thanks for the help!