Hi,
If you know the name of the fields in advance, you can use the lowercase option of the mutate plugin documentation here.
If you don't know the name of the fields in advance, use a ruby script should be the best solution.
A code already exist here. If you want to remove jsonPayload. from the beginning of all fields, edit this line of code event.set(k.downcase, processObject(v)) to this
event.set(k.downcase.gsub(/jsonpayload\./, ''), processObject(v))
event.remove(k)
(the second parameter of gsub is 2 single quotes)