How can I change SFR (bit) address definition when program is running?

In my application, I want to easily change address of SFR include user's commands. Such as:

sbit ram_en=P1^0;
......
/*In another place, I want to change ram_en to P1^2 dynamiclly.*/

I am try to do this(not definition field) in my code:
...
sbit ram_en=P1^2;
....

But it can not pass when compiling.
So, how can I implement this function?

Best regards!