Since the time that I asked the question, I've discovered that sour tomatillos will turn sweet if you leave them in the refrigerator for months. I'm not sure if they'll turn sweet outside the refrigerator, but they will stay good for months outside the refrigerator (at least the variety I tried outside the refrigerator, which was Amarylla; I'm not sure of the variety I refrigerated, but it was green and large).

Since the time that I asked the question, I've discovered that sour tomatillos can turn sweet if you leave them in the refrigerator for months. I'm not sure if they'll ever turn sweet outside the refrigerator, but they will stay good for months outside the refrigerator (at least the variety I tried outside the refrigerator, which was Amarylla, which was still sour after a really long time, but I could have waited longer, perhaps; I'm not sure what the variety was that I refrigerated, but it was green and large).