Thanks---having some follow up troubles. Using your code (as follows) I get no errors but nothing is being inserted anymore. What did I miss? I have a condition for insert and update since with an insert looking at the old map was throwing errors.
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trigger LeadCreateCampaignMember on Lead (after insert, after update) { 
String prefix = CampaignMember.sObjectType.getDescribe().keyPrefix;
List<CampaignMember> cmItems = new List<CampaignMember>();

for (Lead UpdatedLead2: Trigger.new) 
{
      
    if(Trigger.IsInsert){
    if (UpdatedLead2.campaign_id__c!=null && UpdatedLead2.campaign_id__c.startsWith(prefix))
     {
          CampaignMember cml2 = new CampaignMember();
          cml2.campaignid = UpdatedLead2.campaign_id__c;
          cml2.leadid = UpdatedLead2.id;
          cmItems.add(cml2);
     }    
    }  
    if(Trigger.IsUpdate){
      Lead PriorLead = Trigger.oldMap.get(UpdatedLead2.ID);
      if (UpdatedLead2.campaign_id__c != PriorLead.campaign_id__c && UpdatedLead2.campaign_id__c!=null && UpdatedLead2.campaign_id__c.startsWith(prefix))
     {
          CampaignMember cml2 = new CampaignMember();
          cml2.campaignid = UpdatedLead2.campaign_id__c;
          cml2.leadid = UpdatedLead2.id;
          cmItems.add(cml2);
        
     }}
     Database.upsert(cmItems, false);
}
  
}

Also--and I apologize for this, my old test class had 100% coverage. Changing over I drop down significantly. How does one adapt a test class to be better for bulkification? this is my current test class (was 100%).
 

@isTest 
private class LeadCreateCampaignMemberTest {
    static testMethod void validateLeadCreateCampaignMember() {
//Logic: Create a lead. A short coming here to be corrected later is the declarative for OwnerId
        Lead lead = new Lead();
        lead.LastName = 'last';
        lead.FirstName = 'first';
        lead.Company = 'Company';
        lead.Status = 'Unqualified';
        lead.IsUnreadByOwner = True;
        lead.OwnerId='005G0000001Dabm';
        lead.campaign_id__c = '701G0000000Az8r';  
        test.startTest();
        insert lead;    
//Logic: Find the event that was created as part of the lead being created
        CampaignMember cmTestInsert = [SELECT Id, campaignid, leadid FROM CampaignMember WHERE leadid = :lead.Id AND campaignid = :lead.campaign_id__c];
//Logic: Date Time is set to an always true value. Setting it to NOW or similar. Other comparisons are per the Trigger Code        
        system.assertEquals(lead.id, cmTestInsert.leadid);
        system.assertEquals(lead.campaign_id__c, cmTestInsert.campaignid);
//Logic: Check the later half of the trigger--on Update.
        lead.campaign_id__c = '701G0000000BN8Z';        
        update lead;
        CampaignMember cmTestUpdate = [SELECT Id, campaignid, leadid FROM CampaignMember WHERE leadid=:lead.Id AND campaignid=:lead.campaign_id__c];
        system.assertEquals(lead.id, cmTestUpdate.leadid);
        system.assertEquals(lead.campaign_id__c, cmTestUpdate.campaignid);
        test.stopTest();  
        
  
    }
}