Each game fork has its own rules. Additional forks may be possible if the particular game would allow it at the time. Reunifications must be legal in all affected forks.
[Blob] If I may be so bold as to answer on matt's behalf... this isn't Acre Street as such; we're free (or as free as we can be under the usual T&Cs) to bifurcate and reunify games as applicable. So in this case Reverse Chalk Farm and Boardmans have been unified into a hybrid game (Fork Charm 48), which in turn has had the obvious consequence of mutating straight limericks into Limacres. Hope that makes sense.
Incidentally, I intend to make a move 'Real Soon Now', if I can just arrange with Dobbin the Pantomime Horse's agent a suitable fee for a guest appearance.