trigger RecordOwnerChange on user (after update){
if (trigger.isafter & trigger.isupdate){

   List<Id> recordIds = new List<Id>();

  List<account> aclst = [select id, ownerid, owner.isActive, owner.Managerid from Account where OwnerId IN:trigger.new];
  for (Account ac: aclst){
   if (ac.owner.isActive == false && ac.owner.managerid != null){
    recordIds.add(ac.id);
    }
  }
 if(!recordIds.isEmpty()){
  RecordOwnerChange_4Handler.updateaccounts(recordIds);
    }

CLASS:
public class RecordOwnerChangeHandler {
@future
 public static void updateaccounts(List<Id> recordIds){
 List<Account> updateacclist = new List<Account>();
 for (Account ac:[select ownerid, owner.Managerid from Account where Id IN:recordIds]){
 ac.ownerid= ac.owner.managerid;
 updateacclist.add(ac);
 }
 if(updateacclist.size()>0){
 update updateacclist ;
}}}

@IsTest
public class testclassownerassignmanager{
 static testmethod void metest(){

 User u = new User(
  Alias = 'testu',
  Email = 'test@example.com',
  EmailEncodingKey = 'UTF-8',
  LastName = 'Test',
  LanguageLocaleKey = 'en_US',
  LocaleSidKey = 'en_US',
  TimeZoneSidKey='America/Los_Angeles',
  ProfileId = 'Salesforce',
  UserName='test@example.com',Managerid='Test Admin');

  account a = new account(Name='Test');
  a.ownerid = u.id;
  insert a;

  if (u.isactive == false && u.managerid!=null)

  {
  a.ownerid=u.managerid;
  }
  update a;
  test.starttest();
  if (Test.isRunningTest()) {
      System.runAs(new User(Id = Userinfo.getUserId())) {
       update a;
      }
    } else {
       update a;
    }
  test.stoptest();

 }}