I'm assuming you mean mass conversions between *Pantheons*, not just cults - under the Lunar Occupation, the worship of Barntar "replaces" the worship of Orlanth, but because they are both in the same pantheon, most of the same rituals etc are used.
In the case of a Heortling clan converting to, say, the Lunar religion, we don't have rules, but I imagine that it's probably something like this:
For the first few years, a bonafide priest of the new religion had better lead the ceremonies, because no-one in the clan knows them. This may take as long as a generation to get the proper rites known in the clan (ie, a child grows up in the religion and becomes a priest, and then can lead ceremonies). The Old Pantheon will try to "bring their people back to the fold" by sending agents of reprisal - and some high-ranking members of the old religion are going to be in a world of hurt, since gods *hate* to lose priests and devotees and some AoR's tend to attack 'til death. The new religion might (or might not...) be able to protect their new converts. You'll probably see a lot of high-end religious types leave the clan in order to keep their magic.
The Clan/warband/temple wyters will leave unless *major* heroquesting convinces them that the new religion is cool, so there will be a wholesale reforging of communities - the Clan Ring, the temples, warbands, etc. A lot of accumulated magic will be lost (wyters hold all those heroquest goodies that you went questing "for the community"), so a newly converted clan will be pretty weak magically for a few years. Of course, the Lunars are probably *happy* to house a bunch of priests in the clan lands (oh, and of course the priests need guards...).
Of course, it's easier if you can show that the new pantheon has a lot of the same gods ("you worship Ernalda, *we* worship Ernalda"), or that the new gods are really the old gods under a new name ("Look, you call the sun 'Elmal', but it's really just Yelmalio").
Unless a scenario comes up in one of the story-arc books (like Sartar Rising), I doubt that you'll see actual rules for this sort of thing. This is big, BIG magic, and while HW can cover it mechanically, there are probably not a whole lot of times that player groups will need to use it. It probably takes several Xw3 Capital-H Heroes (and/or Xw4 Superheroes) to pull it off. A single Lunar Missionary probably won't be able to convert a clan in a year, it will take a lot of support from the lunar religious types (She might be able to start them on the road, though).
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