this is not possible, because it makes not sense!
Why? Imagine, you have wrote tons of code like this:
FOR i=1 TO 20
Then you Change the length unit from "m" to "mm"!
This change the MEANING of several numbers (the bold ones) in the script.
Now 0.1 mean no longer 0.1 m -> it's now 0.1 mm !
It would be a horrible thing to change all numbers in the script to fit the new unit!
However, you can do this with resetting the smt, and overtake all (see Change_dimension.png)
... but it's a good idea to decide the unit before start of Scripting :-)