Dual stating something as small as a typical Mission shouldn't be much time, but there are several. Dual stating future missions and retro fitting ..... 12 SRMs and ?16? CMPs could be done to let Sixth Edition characters play the earlier NT Missions.
It would complicate setting up Missions at Cons and such as you'd have to specify which edition which could lead to some player confusion, but I think it's better to have the option to pick an edition to finish up the NT campaign. Folks can either play it out with 5th edition rules, or start new characters in 6th and play through the entire arc.
Heck, you could let Players transfer Mission Rewards to alternate edition characters. Player shows up with a 5th edition character for a 6th edition game, oops. Here have a pre-gen, play the game, go ahead and have the rewards for your 5th edition character and toss the pre-gen.