It is unfortunate that you feel you need to make a move since you have characterized your relationship with your current employer as being a good one. Are you absolutely positive that you cannot get more international work at your firm? Have you spoken to anyone about this issue?
The other issue I need to point out to you is that you are a first-year associate and I hate to see you make a move so early in your career. Is there any way you can stay at your current firm for at least one more year? By the way, as a first-year attorney the opportunities available for you are not going to be as plentiful as they might be in a year or two from now.
However, assuming that you have explored all of these avenues and are really certain that you want to make a move, I can tell you that your current employer is most likely not going to be too happy with you when you give notice. I know this may be hard for you to believe, but law firms do not make a profit on first-year associates as a rule. This first year is the time to be trained and to learn how the practice of law is conducted. You are not really a profit center at this point. I bring this up because you are wondering how you can make a move without souring your good relationship with your current firm. Well, I am not certain that you can depart without there being some hard feelings.
On the other hand, if you are not living up to their expectations, they will be more than happy to see you move on without them having to ask you to leave. However, I suspect from what you have told us that you are enjoying a very positive first year. Therefore, I think you need to expect some unhappy good-byes, particularly if you are going to a firm in the same city. You can expect a much more understanding departure if the move is due to a change in geography.
In your particular situation you might have a bit of an easier time because you are looking for a practice area that seemingly is not offered by your current firm. When you do receive a job offer that you intend to accept, it will make your life that much easier if you are making a move because the new firm can offer you something that you cannot get from your current employer.
If you do end up moving to another city in addition to changing practice areas, there probably will not be any bad feelings at all, just sadness to see you go. However, no matter how much you value your relationship with the people at your firm, it is important that you keep in mind not to accept an offer simply because it will preserve a relationship. If you do make a move this early in your career, it is imperative that you do it because you know this is the right place for you to go to work.
Once you do accept a new position, the preservation of your relationship with your soon-to-be ex-employer will be determined by the way you give notice. Make certain that you fully explain that your reason for leaving has nothing to do with your experience at this firm. In fact, your experience has been wonderful. However, you have determined that your true interests lie in the area of an international practice, and unfortunately, this is something that the firm cannot offer. Emphasize your positive experiences with the firm and that you hope you will be able to stay in touch. You might also let them know that you want to make sure that you are able to preserve your relationship and hope that they all feel the same way.
Again, please make sure that before you start a job search in the middle of your first year of practice that you have explored all of the alternatives with your current firm. There is always the chance that you won't have to make a move just yet! Good luck.
Summary: Learn how you can successful move to a new firm and still maintain a good relationship with your current firm in this article.