We are looking for a way to capture the initial priority of a ticket.  For example if a ticket is entered as 'High', but is subsequently lowered to 'Medium', we'd like a way to track that it was initially higher (or in some cases lower).  

I understand how to add a custom function to add a tag to a ticket, but I'm not sure how to capture the priority value on creation, or when to execute the function.

Thanks so much for any advice you can provide!