What other time are you talking about? Bob only mentioned Halloween as a time when a mantle can undergo change in its nature. Yes, the Winter Knight mantle can be handed off any any time of the year, but we are talking about changing the nature of the mantle & maybe permanently fusing it with the mantle of the Warden.
Yes, there might be other conjunctions where an immortal can be killed, such as the stone table, but changing the nature of a mantle might be limited to that particular date.
When Lily got the mantle of the Summer Lady it wasn't changed. The change I'm thinking of, is like Odin making the mantle of Santa Claus. One that didn't exist prior. That was more of a fundamental change, than just swapping vessels.
See my previous post concerning the nature of mantles.
A big part of what defines such mantles is the authority and influence they grant the bearers.
This is more true for some than others, of course. For example, the mantle of the Merlin of the White Council grants immense power to the one who wears it. It doesn't, however, as far as we know, grant any magical boost.
The power of the Merlin mantle is in the authority of leadership it grants over the White Council. It's in the respect granted the wearer among Accords signatories as leader of the one of the most powerful nations. It's in the fact that its wearer has been officially and unequivocally recognized as the most powerful wizard alive. It's social power, political power, and economic power.
The magical power of Langtry may not be significantly greater than, say, Ancient Mai or Ebenezer (ignoring, for the moment, the Blackstaff), but the mantle essentially allows one individual human to wield the magical power, the influence, the knowledge, and the resources -
of the entire White Council of Wizards.
There are limitations, sure, and he can't just do whatever he wants with that power, because it comes from others -- but that's no different from the mantle of the Winter Knight! Mab wields power over a domain of nature, and vests a measure of that power in the WK, but there are checks and limitations on the way that power can be used, and the granter, Mab, can withdraw it at any time. In the same way, the the wizards of White Council vest tremendous power in the Merlin by submitting to his leadership and working with him to provide a unified front to the rest of the world, despite any internal differences. The Merlin is limited in his use of that power, however, both by design (the checks built into the government of the Council) and by the nature of that power (its use depends on getting a LOT of free-will-possessing people to work together).
My point here is that the question "when can the nature of a mantle be changed?" is the wrong question to be asking. I don't think there's this concrete category of things in the Dresdenverse called "Mantles," following a defined set of rules, in the same way, for example, that "Immortals," "Mortals," "Spirits of the Nevernever," or even "Ghosts" are. I also don't think there's some type of discrete magical construction called a Mantle, as is the case with such categories as "articles of faith" or "greater circles."
A Mantle is a role or a position of responsibility, and everything that goes with it. It's a word that describes something much more than an entity's job, but less than its identity.
The question of Harry changing the Mantle of the Winter Knight is actually asking whether or not Harry can change the portion of Winter's power granted to him by Mab. She gave him a portion of herself, because that is a large part of what makes someone the Winter Knight. When he talks about the "mantle's influence" or attributes dark thoughts and impulses to the Mantle, he's using the word as shorthand for "the portion of Winter's essence placed in me as the Winter Knight." The question that's valid is whether or not the nature of that portion of Mab can be influenced in the same way Lasciel's Shadow was, or alternatively, by extended interaction with soulfire. If you want to use Harry's shorthand and call it the Mantle, fine, but please remember that it's a word, and that it means something specific - not an arbitrary label for a category of things.
Immortals are what can only be changed on Halloween. Maybe immortals can only take up new mantles or set down old ones on Halloween. But that is a function of them being immortals.
