What sort of repercussions, if any, would a Guernicus face in trying to leave his house and joining house Flambeau?
What sort of repercussions wold any True Lineage magus face in trying to join another house?
What sort o repercussions would any Mystery Cult magus face in trying to join another house? I assume they'd be bound not to share their mysteries (or maybe just the Verditius and Björnaer ones)
Societates magi would have it easier, I assume, but Tytali and Flambeau might not leave him alone anyway.
Reading Magi Orbi on page 106 in HoH: Societates may be helpful.
Aside from the legal ramifications (going Orbis means you have a time limit to find a House or you get kicked out of the Order), I would say not that much for some of the Houses. They have an example in Societates of a Tremere who joined Flambeau and introduced a new school of magical combat.
A Guernicus doesn't mean a Quaesitor, it could mean a Hoplite, who would be an ideal fit into Flambeau. It would be the circumstances of how they left that would be an issue. If they burned a lot of bridges on their way out, I would expect there'd be political fallout and possibly Wizard Wars. Keeping the Mysteries might be something that could cause some issues, but that could possibly be secured by oaths and such, most wizards still respect their own word.
I have read the Orbis section in Societates, but I was imagining some houses would not appreciate "deserters", so to speak.
Also, since you mention the issue of Quaesitor, I would assume such magus would not retain his Quaesitorial position, since it is much harder for magi from other Houses to have it. On the other hand, he was trained Guernicus, so that might suffice for the Council to allow him to remain Quaesitor.
Of course, if he was not a Quaesitor in Good Standing to begin with the point is moot.
also look up Dankmar GotF