Can I change my age expectancy withdrawal after retirement?

Under current rules you are allowed to change a life expectancy based monthly payment to a specific dollar amount monthly payment one time. AND, you are not allowed to change back.
The recently approved legislation that deals with TSP withdrawal rules might lift this requirement, but we’ll have to wait and see what happens when it is implemented by the TSP. The TSP was given up to two years to come up with implementing regulations.
If you had chosen the life expectancy based payments from your TSP as a way to avoid the early withdrawal penalty (sometimes called a 72(t) withdrawal), you must continue on life expectancy based payments for the longer of five years, or until you attain the age of 59 1/2, in order to avoid the penalty.