What to do when your research interest changes to a new topic half way into your PHD?
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.