Yes, it will effect your SERP. Title tag has a important role in on page optimization. So, it is better to extend your title to include more keywords.
Yes. Changing your title tag will almost certainly affect your ranking, but you won't know until you do if it will be positive or negative.
According to my own experience every change in your title will result in a drop in SERP's. Every time you change your title spiders have to re-evaluate the new title and its relevance relatively to other established sites. Even if you only add one more word to get long tail it will drop. The drop should be quite small if you add few words and keep the existing key words included. So think twice before changing titles!