Change of System variable sy-uname during the program execution

Hi,

I wanted some help, if it is possible to change  value of sy-uname during the program execution.

I am mentioning the flow of the execution as follows.

Step 1. Intially sy-uname is having the value of the login user, for example say 'user1'.

Step 2. I want to set the sy-uname to 'user2' and after that i call a function or a method. The system should take the sy-uname as 'user2' for executing the function or method.

Step 3. Once the control comes back from the function or method call the sy-uname should be set to the logged in user.

Regards,

Om