Starting 23 April 2013 Apple has locked App information fields in iTunes which means that you cannot edit them while uploading a new version of the app. However folloing feilds are still editable
- App’s support URL
- Marketing URL
- Whats new in this version.
Apart from above mentioned feilds every thing else will be locked. Rationale of such a decion can be debated but this is the reality as of now.