Question About Faction

I went through the game as a Luxon. Now I'm done. I want to do the Kurzick missions so I can unlock some farming areas and Elite Caps. Now, I think I have to do the 10K Kurzick Faction deal to get to the next mission. If I do this will I retain my friendship with the Luxons or will I have to find another way to rejoin them?

If you have completed the Befriending the Luxons quest that opened up new content, you will still have the benefits after completing the Befriending the Kurzicks quest. I would suggest you trade your Luxon faction for Jadeite or contribute it to your guild before beginning to earn your Kurzick faction though, since when you spend faction of one side, you lose an equal amount from the other. If you have multiple characters, you should also have them all complete the quest on the same 10k, it will make it much easier.