Be aware of the following considerations for trigger context variables:

• trigger.new and trigger.old cannot be used in Apex DML operations.

• You can use an object to change its own field values using trigger.new, but only in before triggers. In all after triggers, trigger.new is not saved, so a run time exception is thrown.

• trigger.old is always read-only.

• You cannot delete trigger.new.

Cheers...