However, my suggested steps to introducing nUI into your gameplay without a fresh install is to do the following:
1. Rename WTF folder so that you are using new data.
2. Turn off all addons even nUI and log into virgin wow so that it can create the fresh new data it needs.
3. Turn on nUI on its own and log into a toon you won't need to raid with for a while but enough to test things out.
4. Do some quests, kill some mobs, every day stuff and see if there is something missing that nUI doesn't give you.
5. Turn on the appropriate addon you have been using and make sure it still works with nUI without any compatibility problems.
6. Repeat steps 4 and 5 until you have a running system without raiding.
7. Next look at what raiding addons you need and see if you noticed it as part of nUI proper in which case you won't need them. Others, just install and make sure they don't have loading issues.
8. Repeat step 7 until your much needed raiding addons have been activated.
9. Make a note of the addons you are using and remove the other ones as you no longer need them.
10. Now go enjoy