What is the precedent for a mage changing their House affiliation?
For instance, I have an Ex Miscellenia in my Troupe who, after studying Faerie for several in game years, is wondering if she can join Merinita.
Is this permitted by the rules? Is this feasible for a Mystery Cult or a True Lineage? I imagine almost any mage could join Ex Miscellenia, but beyond that, are there Houses open to admitting guantleted mages from other Houses?
The True Lineages are defined by the fact that all of their members were trained by somebody who was trained by somebody who was trained... by the founder. So no, you can't join those after Gauntlet. For all other Houses, it's a matter of persuading them to let you join. They might want a lot, or they might embrace you; they might be very suspicious, or they might be more inviting.
I don't think there are official rules, but it's certainly implied that magi can move between Houses. Brayan is of course correct, but here is my view on the case at hand...
Specifically to House Merinitia, I think if you prove yourself to have true interest in Faerie and desire to embrace the Mysteries and philosophy of the House (i.e. an adventure or two), the Primus would have no problem letting you join in (perhaps after an interview).
Specifically from House Ex Miscellenea, I think the House will not mind you leaving. Other Houses, such as Tremere, might harbor a grudge and may even attempt to prevent you from leaving or demand some sort of punishment. But Ex Misc shouldn't care.
IMO,
Yair
Or just plain declare Wizard War on you. House-wide. Ouch.
I wonder if there is a provision for "An Enemy of the House" in the peripheral code... 
I'd say that many houses would not accept applicants from houses they know would react badly. No house is going to pick a fight with the Tremere over one measly wizard that they don't care about.
The cult houses especially would throw a fit if their members started defecting. State secrets and all that.
And if the house you apply to join refuses you after you have left your current house, then you have left the order and will be marched by everyone.
If I remember correctly, there are several mentions of magi switching houses in the books - typical examples are magi leaving Merinita when the house shifted focus to a mystery cult and faeries, and some leaving Diene to hide in other houses during the war...
As a matter of fact, HoH:MC explicitly mentions that mages from other houses joing Merinita is not only allowed, but expected and treated rather nonchalantly, to the point that mages that initiate Merinita mysteries but do not formally switch houses might even not get severe social repercussions (i.e. no Wizard Wars) beyond the cold shoulder. It appears that as long as you have the interest in, and correct mindset about magic, faerie, creativity, and nature, and enter the fae Mysteries, you are assumed to be one of the Boys and Girls, membership cards notwithstanding.
This relaxed attitude is one of the reasons why Merinita is one of my preferred Houses. 8)
the fun things happens when a Bjornaer, privy to the deep/dark secrets of the Tribe, acquires Faerie Magic somehow, and finds the whole Order of Hermes (with the violent exception of Bjornaer!) expects the faerie-magus to do the right thing and switch Houses, but his tribe will take it ... badly ... if he goes off with their secrets!
It is my opinion that there is a social hierarchy amongst the House groupings -- True Lineages, Mystery Cults, Societates.
True Lineages are only open to those directly of the Lineage, stretching all the way back to the Founder of that Lineage. Thus all Bonisagi may, ultimately, trace their training back to the Founder Bonisagus himself. Since this is a closed group, there is a certain status attached to it -- no one may join these groups after the fact, thus they are more exclusive.
Mystery Cults have entrance requirements. Each Mystery has a different requirement; in the case of House Bjornaer there is the obvious requirement of being allowed to join in the Gathering of Twelve Years. The other Mysteries are a little bit vaguer that way from what we know, but it is easy to extrapolate points. Still, there are definite requirements -- you must give up something to gain, the core concept of all Mysteries. These requirements do not stop members from the Societates (or, potentially, other Mysteries) joining this Houses, though they have to put in an effort.
Societates are the least exclusive. In fact, in the case of House Ex Miscellanea, there are almost no restrictions and it might even be the case that if you belong to no other House you belong to Ex Miscellanea (this depends on how you read many different passages). Since these are very much non-exclusive (you only have to generally agree with the aims of the House which, in the case of Ex Misc, is minimal indeed) there is little social gain to being a member of one of them.
Therefore the hierarchy is reinforced -- True Lineages, Mystery Cults, Societates. This would also suggest that over time the number of members of the True Lineages might well shrink, while the Societates would swell; certainly in the case of Ex Miscellanea we see that they are the largest House.
Anyway, concepts to ponder.[/i]
A fun plot point just before my table-top game ended was the Bjornaer-Jerbiton war, after a nobel born Bjornaer defected to Jerbiton. Fun stuff.