Yes she is. I transplant them on any stage, so don't be scared by internet myth. Use mycorrhizae when transplanting, sprinkle on roots and into hope, roots and plant will love it.
Yes that plant is flowering, i saw in your profile that you thought it was showing her sex, but no, she is flowering already, is it the watermelon feminized or is it auto watermelon, maybe thats the reason, not sure?