Python 3, pxeger
With direct memory access, nothing is really immutable. Replaces the string in the constant table of the lambda function.
import ctypes
old_constswow = f.__code__.co_consts[2].co_constsco_consts[1]
new_constshah = (None, 'hah')
ctypes.memmove(id(old_constswow), id(new_constshah), new_constshah.__sizeof__())