As far as i know you cannot change the order of the fields.
What would be the reason to change the order if i may ask?
I already submitted a wsdl to the party that is going to consume it, I then decided to do some rework in my model and use generalization so I could reuse those parts of the message that are common to all messages (source, interface, sourceID e.g.). These fields where in the beginnen part of the message before generalization (ordering the attributes in the entity did that) but now aren't. Beside it being extra work to issue a new wsdl, I like these fields to be the first fields in the message because it's neater that way (according to me that is).