To start please post in the correct forum next time :)  I think most look at the full list. 

Back to your question, 

You can have a service item as a template and clone it. How do you mean replace it?
Keep in mind if you delete a field or hole service item all ticket data is removed. 

What you can do is to use drop down and source a custom object with a filter on a field in the custom object. ( you need pro license)