Preserve Current Paths before rolling out new Pathauto Alias
To preserve AND redirect existing aliases when you implement a change like that:


install the Path Redirect module
open the 'Automated Alias Settings' in URL Aliases (admin/build/path/pathauto)
open the 'General' fieldset
change the radio button for 'Update action' to 'Create a new alias. Redirect from old alias' (see screenshot below).


http://img705.imageshack.us/img705/2738/redirectaliasleaveintac.png