Unfortunately, once a plant starts to develop seeds, it is past its prime for culinary use. We likewise let some leeks overwinter but dug them soon after they began growing again this spring. We did let two develop seed heads as we wanted to gather the seed and try to propogate from those seeds next year.

You can cut the scapes while still tender and use them just like leeks.
Also, let a few to go for flower/seeds. Leeks' flowers are very pretty and
you can let them mature and then collect your own seeds. That is what I am doing.
You can leave the ones you cut the scapes off and pull them in two to three weeks.
They will have a garlic like bulb, but very SWEET, WITHOUT GARLIC SMELL.