My experience is that sunburn affects the leaves that were sunburned but not, necessarily the whole plant. I have some seedlings that were severely sunburned and still bear the evidence with red leaves and black spots - ver ugly. But the plants have grown new leaves and are doing very well now that they get proper light. I suppose you may have overdone it, but I would just get them to the proper light and see what happens. Keep watering as normal unless you've killed off the roots also.
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