Sunflowers are sensitive to root disturbance. You can do it, probably with some fatalities. Gently dig out the plants trying not to disturb the soil around the roots. If you have close clusters of seedlings, dig them out as a group, don't try to separate them, transplant to their new location, then snip all but the strongest one at soil level with scissors.
However, they sound a bit tall for good future development...I'll bet they are quite slender and leggy. If you have any more seeds left, you might have better success starting new ones. They germinate quite easily well into the summer.
