Hi Mohan,
You are right from documentation "Changes on the Runtime panel will apply immediately" seems to be posible to change without restarting but until my knowledge and reviewing WAS console you have the same message " Changes on the Runtime panel apply immediately." but from runtime tab you only can review the logs.
If you use HPEL mode the logs are binary and you need to use logviewer, even that you can enable HPEL text log.
I have done the test to change in runtime HPEL text log and works after uncheck and check "Enable text log" in runtime tab. I have change the directory not the name of the file.
So seems that you need to switch to HPEL mode to get this feature.
Hope this helps. Tell us if you need more support.
regards