depends...
it seems the cult of the dragons option is disallowed for version 2.25
otherwise ..
maybe ue purge of the unfaithful, then allow for natural spread of dragon cult, competing with other religions ? (as there are 3 free religion slots in each city)
otherwise... isn't dragon cult spread by the great dragons ?? if the great dragon acts as a holy city, moving him from places to place should locally increase the spread rate as it would be equivalent to moving a city closer to the holy city....