I would tend to assume that the maintenance team changes are relatively small and get integrated over to the main team on a fairly regular basis - if not they are in for a whole lot of fun come the end of next year.My question to FD would be if the New Era uses the bug fixed branch of the game (i.e. the current build), or if its a 'new' branch built to cope with Legs, atmospheres etc. If its the latter, what can be shared if New Era is still in flux? Conspiracy theorists might then suggest FCs are underbaked and bugs offer a reprieve in the 'Old Era' branch, with the Letter being a good excuse. My concern is that if New Era is an independent branch its that which needs testing to the max, and not the current build which would in theory have a year of operation left to run.
Presumably this gets harder the further the main team move on but probably better than the alternative. A lot of this may come down to just how modular stuff is, no idea.