Will "Overwintering" Onions Grow Now?

Sadly I missed the boat with planting them to overwinter so I planted them in March.  My gardening 'know-all' friend says that I have wasted my time.  He reckons they'll bolt and not bulb up    Is that so? 

I'm wondeing if I should pull them out and plant with something else?

Onions won't bolt until they have gone through a winter with enough size.  A small seedling onion won't bolt following a winter, but a larger, older onion plant will bolt in the spring following the winter.  Around here (South Carolina) many gardeners transplant onion seedlings into their garden in late winter, where they grow, bulb up, and go dormant in the summer.  If left unharvested, it will come out of dormancy in the fall, grow through the winter, and bolt, set seed, and die on the following spring.

Ok, so leave them insitu?  I understand from your post basjoos that they're currently dormant and will be ready to harvest in the spring after growing through the fall/winter.  The white onions seem fine but the red ones are beginning to form flowers - is that bolting?

my multiplying onions are just now producing the baby bulbils that will be ready for replanting soon, but i'm not sure you are talking about multiplying pere nnial onions or something else.