All the information I've seen about this expansion so far says that you replace the Genestealer deck with the Tyranid one. Does anyone know if it's possible to simply combine the original deck with this one so you never know if you'll be facing Tyranids or regular Genestealers in each spawning?
The two decks have different backs (purple for Tyranids), so you'd know a little about what was coming next. The textures are also quite different so unless you sleeved the cards you might find they tended to sort into bunches when you shuffled them.
Other than that, there's nothing to stop you combining the two, and if you found a pure Tyranid deck too difficult, diluting it with a bunch of Genestealers would make things a bit easier.
Around half of the Tyranids are normal ones which behave exactly like normal Genestealers, so you do get the "not knowing what's coming" element anyway.