Is the check determining if there is a valid target for the effect made as if the card is transformed into that new type, or as the old type.
Examples include: Bannermen, the event ones use a "then" clause which implies "if able" so the "valid target check" is moot for the attach part.
But that is the question, so far it seems that the flavor of ruling right now is if the effect transforms a character or event into an attachment, then treat immunities when determining targets as if the card is an attachment. Not only that but the attach clause of a triggered effect is not considered part of the triggered effect.
Also, did something change with the immunity keyword? Last I remember if something was immune to triggered effects it meant triggered effects regardless if it chose the character or not. (Jorah, Westeros Bleeds)... however a octgn game came up where apparantly rusted sword can use it's challenges ability and attach itself (as per triggered effect) to a character immune to triggered effects. And if so... does this also mean the events that become attachments... can they attach themselves to a character immune to events (as part of the then clause).
Also is it still the case that targeting uses "choose", meaning Eddard cannot cancel something if it does not have the word "choose" in the text of the effect.
(This is also less clear because cards that do infact allow choosing such as kill a participating character, or kneel an opponent's character imply choose but do not say choose).