It is a very common question and dilemma that lingers in the minds of candidates thinking of entering in UPSC exams. The subjects should be well decided before it is too late. A wrong choice will haunt you forever and thus take some time and explore your choices, so that first step is taken and placed firmly. Well begun is half done. Any doubt regarding subjects chosen will not guarantee any success. Choose subjects ahead of time. This is because this is just the first step. Consequent steps will be to select best of course material for the subjects you chose and then forming the study group which will ensure better grasping of material.
You need to select the subject with which you are familiar with. This will save a lot of time. If you are going for a complete new subject, then you will be spending a lot of time on familiarizing with it. Thus, most of the UPSC candidates choose a subject which they have opted in their graduate levels.
Some of the candidates like you are not interested in the subjects they chose in their graduate levels but find other subjects amusing. Like in your case, you are thinking of political science. It is okay and perfectly fine to opt for it, but do not make a haste. Go through the study material. See whether you can cope up with it. Even though it is new for you, but can you familiarize with it and hold up the interest level as you have to spend lots of time making an extensive preparation in it.
Also, choosing a subject which does not have too many candidates for it is a wise idea as you will be cutting your competition. Irrespective of whether you chose political science or subjects from your graduation, you have to keep in mind that you will be competing against lots of candidates and thus less the competition is more the chances of success you will be favored with.
Go through previous years question papers, discuss with people who have previously opted for the UPSC exams and then decide. Analyzing past trends and present competition are very necessary. Blind faith can mislead you. More and more you discuss with others, more open you will become for the options.
It all depends on your interest and the number of hours you are willing to spend on your chosen subject. The more dedicated you will be the more knowledge you will be grasping on that subject matter, no matter how unfamiliar or familiar the subject is to you. Ultimately, it is you who has to decide as you know yourself better than anyone and thus based on your liking and interests, calculate everything after going through all its pros and cons and take a decision.