Source: http://forums2.gardenweb.com/forums/load/cornucop/msg031625314072.html
Leeks transplant very well, and it's early in the season yet. You could just dig them up where they're too close together. Put some back and maybe transplant some others to another location. I grown mine about a foot apart and they grow quite large and sit thru the cold winter. Even if you want yours only 6 inches apart, it should be easy enough to just rip out the spares if you don't feel like transplanting.
And if you do transplant any, you can do two things.
1) Plant them a bit deeper to produce a larger, white, edible portion of the plant.
2) Put compost or other organic fertilizers beneath the root zone so that the roots can grown down and feed. Leeks are heavy feeders.
Heck, you might just dig up the whole lot, and replant at your preferred spacing in a nice prepared, amended ditch.
That's how I grown leeks. I just start them crowded together in an unused corner of the garden somewhere, and then in early July, I dig them all up and transplant them to a nice prepared row. Crowding them when they're young is no problem. You might wait until their stems are about a pencil's thickness before any transplanting. But thinning can be done at any time.