You can move email to another retention folder (i.e. from your 2-year folder to your 7-year folder).
NOTE: If you move an email to a retention folder that is a shorter time frame than the one you are in (i.e. 2 years to 6 months) the retention will apply immediately. If the mail you were moving was a year old once you moved it to the 6-month folder the vault would delete it.