AFAIK, if you already own your Nuendo 6 license, you'll have no other choice than to buy a new NEK 6 license.http://www.steinberg.net/en/shop/buy_product/product/nuendo-6-expansion-kit.html
Upgrade path is possible for Nuendo or Nuendo+NEK but seems not available for NEK alone.
To be confirmed by Steinberg.
Nuendo 6.x, Cubase 7.x, Wavelab 8.x
PC A:XP Pro SP3 & Win7x64 / GA-P35-DS3P QX6850, 4G DDR3 / 2xRME Digiface / 2xPowercore PCI MkII & Firewire / MCU Pro
PC B:Win7x64 / GA-880GA-UD3H X6 1090T, 8G DDR3 / 2xRME Digiface / 2xPowercore PCI MkII & Firewire / MCU Pro
PC C:Win7x64 / P5QSE Q9650, 8G DDR2 / RME Digiface