That's actually what I brought up today with some friends, a two-month release for 'Nids makes sense if they are going to re-do that much stuff in plastic. It also makes me wonder where new units will fit in, as I doubt they would solely leave new units to being options in dual kits. Look at this;
Tyranid Prime - plastic clampack
The Parasite of Mortrex - plastic clampack
Tyrant Guard/Hive Guard - plastic dual kit
Lictors w/ options for Deathleaper - plastic kit, maybe dual kit with new unit?
Zoanthropes/Venomthropes w/ options for Doom of Malan'tai - plastic dual kit
Pyrovores/Biovores - plastic dual kit
Genestealers/Ymgarl Genestealers - plastic dual kit repackaged for Ymgarl bits
Tyranid Warriors/Shrikes - plastic dual kit repackaged for Shrike bits
Mycetic Spore - plastic kit
Ripper Swarms/Sky Slashers - plastic dual kit
Harpy - plastic kit
Spore Mines - plastic kit
Carnifex - maybe redo the instructions so that Old One Eye can officially be made out of it (there is already a head for him in the kit)
The Red Terror - What happens here? Becomes out of production maybe?
11 plastic kits and 2 clampacks before ANY new units are added in! It's why I personally don't see all their Finecast stuff disappearing, I expect that Spore Mines, Rippers, Lictors and Deathleaper at the very least will stay Finecast, but maybe more. Still, that they need so much stuff to be redone/given models before even considering new units is just crazy. That's why I actually am starting to genuinely think they will get a two month release. With a new (hopefully good) codex in tow, it would be the perfect way to massively kick-start interest in Tyranids again.
As far as new units, given that we've seen an entirely new base size for Centurions, maybe those early rumours about a giant Tyranid monster aren't so out of whack. I personally don't see it though as the Harpy and the Mycetic Spore would probably fill the "big new kit" quota anyway, but it is nonetheless a thought to entertain.