Think of it as immutable like a string is immutable.  The cases are exactly parallel (the string function is of course named 'replace' since it doesn't have to deal with the 'arbitrary attribute names' problem namedtuple does), except that it is much easier to address the parts of a url using the namedtuple.

_replace is not a "weird secrete method", it is part of the public API of namedtuple.  I agree that using '_' is unfortunate.  I would have preferred a name pattern like _replace_, to make it clearer that it is *not* a private method.