How to avoid Exeception on After Insert, Record is read only

Trigger will be like,

trigger AccountMasterTrigger on Account (before insert, before update, after insert, after update, before delete, after delete) {  
    //AFTER INSERT
    if(trigger.isAfter && trigger.isInsert){
    //query the accounts to update otherwise you will get read only error.
         List<Account> acclist=[SELECT type,Group_Type__c,Physical_Zip__c,Physical_Address_County__c,ownerId FROM Account WHERE Id IN:Trigger.newMap.keySet()];
         AccountOwnershipUpdate.change(acclist);
    }
}



Helper will be like,

public class AccountOwnershipUpdate {
    public static List<Account> accstoupdate;
    public static Zip_Code_Region__c zcr;
    public static void change(List<Account> acclist){
accstoupdate=new List<Account>();      
        for(Account a:acclist){
            if(a.type=='Prospect' && a.Group_Type__c=='Small'){
                try{
                    zcr=[SELECT Id, Name, Zip_Code__c, Small_Group_Sales_Consultant__c, County__c
                         FROM Zip_Code_Region__c
                         WHERE Zip_Code__c=:a.Physical_Zip__c
                         AND County__c=:a.Physical_Address_County__c
                         LIMIT 1];
                }catch(QueryException qe){
                    System.debug('Unable to query zip code region due to '+qe);
                }
                if(zcr!=null){
                    if(zcr.Small_Group_Sales_Consultant__c!=null){
                        a.OwnerId=zcr.Small_Group_Sales_Consultant__c;
                        accstoupdate.add(a);
                    }
                }
                zcr=null;
            }
        }
        try{
            update accstoupdate;
        }catch(DMLException de){
            System.debug('Unable to update account due to '+de);
        }
    }
}