Yes, you can change the ordered period in the license details section, by clicking on Upgrade. The new period starts at the moment of the next renewal of the license.
- April 1, 2012: you order a license for 6 months; the expiration date is October 1, 2012
- June 1, 2012: you change the ordered period to 1 month; the expiration date remains unchanged at October 1, 2012
- October 1, 2012: the license needs to be renewed, and this is done for 1 month; the new expiration date is November 1, 2012