I have an (after insert, after update) on Opportunity. I monitor the (AccountId) for changes.

trigger updateOpportunity on Opportunity (after insert, after update)
{
    Set<Id> opiIds = new Set<Id>();
   
    for (Opportunity opp : trigger.new)
    {
        if (System.Trigger.isInsert) {
            oppIds.add(opp.Id);
        }
        if (System.Trigger.isUpdate) {
           if (Util.hasChanges(fieldsTracked, opp, trigger.oldMap.get(opp.Id))) {
           oppIds.add(opp.Id);
           }
        }
    }
    if (oppIds.isEmpty()) {
        return;
    }
// Do some stuff

If I physically move the Opportunity from one Account to another, the trigger fires as expected. But when I merge two accounts, the opportunity thereotically changes it's parent association to the new AccountID, and my trigger monitors changes to this filed, but the trigger doesn't fire.

Is there a function to monitor a merge differently than an update? trigger (after merge) isn't accepted, and (System.Trigger.isMerge) isn't accepted either.

Yes the Master is the new Account, and the Opportunity is associated to the old Account which gets deleted after merge. So one would think that the Opportunity is getting updated with a new AccountId, but if (Util.hasChanges(fieldsTracked, opp, trigger.oldMap.get(opp.Id))) still produces 'FALSE'.