Run a year end on September 30th?

Changing year ends is tricky, and you will almost certainly need special financial statements to make you comparatives work for the short year.

You can always just change the dates in the fiscal calendar, but that doesn't adjust the periods that the transactions have already been posted to.

If you have posted some transactions in August, did you want them to be in period 8 or period 11?

I would strongly suggest getting expert advice to assist you in all the ins and outs of changing your year end. It's not a trivial operation.