I do not have a huge amount of experience growing Neps from seed, but I do have a good deal of experience with acclimating many other genera of plants to a wide variety of settings. In fact, I just went through hardening some awesome Nep seedlings from Whimgrinder to the conditions of my greenhouses at work. The process took about 3 weeks, and he can probably remember better how old they were at the time (not at work to check the labels)... they vary in size but are generally 1" in diameter. When I got the plants, they were bare-rooted. I immediately potted them, fed them lightly at 1/16 tsp per gal strength to stimulate the formation of root hairs. If they lack any nutrition at all root hair formation will be limited. Good feeding minimizes transplant shock.
Next I put them into a humidity dome and watching them meticulously. After about a week I began propping it up more and more (punching holes is also recommended, but remember you can't easily reverse this!) until I saw no desiccation. Things like the pitcher lids closing and the leaves losing turgidity were my signs to keep the humidity high. It took 2-3 weeks for them to tolerate the new lower humidity conditions. I never lost a single plant out of four different taxa.
I would absolutely avoid repotting unless you need to. It adds a needless additional layer of difficulty because you're doing root damage to the plants. To acclimate to lower humidity, they need every root hair they have and often need to manufacture more.
For the best survival rate, two things are the most important:
1) do it slowly. That's why I noted that it took about 4 weeks.
2) Ensure the plants are as healthy as possible before you begin this process.
Good luck! Keep us updated on em!