What to do when your research interest changes to a new topic half way into your PHD? I had started my PHD last year on a topic that has several interesting problem to solve in the area, however I don't find the problem interesting enough to spend 4 to 5 years. As time went by and I came across a new problem through my course work and interactions with various professors. I have started liking a different topic that is not very related to the original. 

What would be a wise thing to do in such situation? Is it a common situation (changing topics midway)?
First, I've yet to meet a single PhD candidate that actually did what he proposed in his research plan in the first place. Unless he entered for some particular project (and still)

Is not uncommon to look for different topics and think that it may suit you better, and in all fairness, you should be doing something you like, not some topic other person imposed on you.

Now, switching topics, specially if they are unrelated, will have the consequence of delaying your PhD graduation considerably. I switched topics on my PhD 2 year, but I mostly changed the application, while the most fundamental part (in my case, it was the math) was pretty much the same, so I got to use most of the foundations I learned over the first couple of years.

I hope it helps.