Thank you Dave. Is below variable distribution valid? Or do I have to change distributions via variables by overriding pre_randomize function?
constraint keep {
if(!value)
value[1:0] == const'(value[1:0]) dist {1:=5,0:=95};
else
value[1:0] == const'(value[1:0]) dist {1:=95,0:=5};
}