Until it all results in a loyalty cascade when some guest happens to join the were due to loyalty (or whatever), and that guest has chatted up some militia member...
Or, better still, the necro visitor getting agitated and raising the were, with half the people present then joining on the necro side.
After a few more "controlled" accidents, I found that soldiers attacking werebeast guests don't cause loyalty cascades. Wereguests seem to stay neutral and friendly toward citizens and other guests until they were spotted by an active military
dwarf (or maybe any active soldier belonging to your fort?), at which moment their brains finally realized "oh wait a minute I'm supposed to kill everyone" and does so. Visiting warriors and monster hunters do not trigger it, but bodyguards (diplomats, merchants, etc.) seem to. De-transformed guests that had gone hostile will remain in the other faction as well, as I saw when the (new) human diplomat transformed into a werebeast and killed a dancer while his bodyguards were watching behind fortifications. The diplomat was later killed by his own bodyguards, but the bodyguards remain friendly toward everyone else, including active military units. The human civilization that he's from also does not count this as an act of war, apparently?
Socializing guests that transformed while talking to an active/training soldier does not trigger a loyalty cascade, probably because the soldier's activity never officially turns to "socializing." No proof has been observed on whether they remain socializing while in combat. Further research required.
non-intelligent undead stuffs remain in their own "me zombie blergh" faction and does not trigger cascades. Necromancers are weak and pathetic and were instantly bludgeoned/bitten/gored to death almost all the time. Further research required.
It should also be noted that, in the case of wanting to kill "friendly" wereguests, that their faction switches and will turn hostile the moment a "kill" command was delivered, regardless of line of sight. Something I probably should've expected.