Conference presenting: stick to initial paper submission or include recent improvements?
do I have to stick with the work that is going to be published, or can I also present some of the more interesting, newer work?


Absolutely present the improved model. 

Conference submissions photograph the state of the work at the time of the submission. But from the submission deadline to the conference, many months can pass and usually people continue to work on their projects. Frequently, by the time of the conference, new results have been obtained, and these surely deserve to be presented.