So, I put my leeks in the wrong place last fall. I also broke almost every rule of planting leeks, but let's ignore that for now - they survived and are looking pretty perky now, bigger than the "pencil thickness" size that you're supposed to transplant them at, but smaller than the inch-thick or more that they're theoretically supposed to reach. For no reason that I can recall, I planted them at about a one-foot spacing. I now want all that space that they're luxuriating in.
Can I transplant them, and transplant them to roughly a four-inch spacing? My general impression is that leeks are pretty tough and almost enjoy being transplanted, but is there a size where that's really really no longer true and transplanting them is just a slow and laborious way of killing them or inducing them to bolt?
If it makes a difference, one of the things that I did wrong when planting them was mis-prepare the bed; if I transplanted them, they'd have less space but plushier and more nutritious soil.
I grow leeks and let them overwinter. But I have never planted them in the fall, always late spring. I start mine indoors and plant out then they are still quite small but easy to handle. They usually overwinter fine and I love being able to harvest them in the spring well before they go to seed. I don't think transplanting would be a good idea at this point. Fertilize them and let them grow on - they may get bigger yet. Harvest before they go to seed - if a seedhead starts to emerge cut it off and harvest! You might consider planting some new leek between the old or something else. Or plant new leek in the better bed. You are in a completely different part of the country from me. Here the norm is plant in spring, harvest in fall, but I stretch it a bit and have been lucky so far. Leeks require a long period (over 100 days) to mature and sometimes they are rather puny in the fall but get really nice by the spring. You could experiment and transplant a couple to see if they take. Good luck and let us know what you do.
Leeks are like onions. They are rather forgiving. You are still in a cold zone. A lot of us in Texas go back to Dixondalefarms.com when we have memory lapses as they have reallly good info on onions and leeks. If you arent wanting them totayn a whole new bed you could replant tighter, but it will slow them down or even stunt them.