Reassigning records owned by inactive users is quick and easy using the Reassign Module in DemandTools. Simply use the “Record Selection Criteria” to pull back records where the Owner.IsActive flag is false. (See visual below) Once that condition is set, only records owned by inactive users will be returned. Next, just go through the remainder of the steps to reassign to the new owner or owners. The example below is based on Accounts, but this will work with any object. Keep in mind, you have the option to assign sub-objects simultaneously. Here is a quick Reassign tutorial.