IME the thighs won't really stretch at all (had a pair of new cures for awhile). Other people say otherwise so take that as you will. I sized down one in the NS and have been wearing this pair every day for the past week or so and they seem fine so far - the waist is still tight. I expect them to stretch out a bit more and then, once washed, be about perfect.
You also might want to just get another brand.