And another one:
sample /. Map[sample[[#, 6]] -> ImportString[sample[[#, 6]], "JSON"] &, Range@Length@sample]
This will not change the structure. Which your third example does:
Deleted
One alternative:
test = sample; (test[[#, 6]] = ImportString[sample[[#, 6]], "JSON"]) & /@ Range[Length@sample]; test
Is there a way better than
sample /. sample[[#, 6]] -> ImportString[sample[[#, 6]], "JSON"] & /@ Range[Length[sample]]