In AX2009, can we change the production order's end item after the production is started and has picked some materials? In our process, sometimes after we start our production, we found that a small change in the process which will result in a different end item. Is there a standard function to update the end item as well as all the related production postings created already?
It is not possible to do that.
You have to end this production and take the item to inventory.
Then create a production order for actual end item that you want to make and consume the previously produced item into this production order along with other parts.
How often does this happen? If it is too often, i would suggest you bring a Sub-BOM into picture.