Changing the thesis topic and supervisor
By all means you may go for your current research interest. Switching research area isn't uncommon. It will give you a refreshing start with new challenges (it did for me when I switched from data security to machine learning). 

Interest and determination is the prime factor in research that's worth more than knowledge in the field. I've noticed quite a few grad students (both PhD and masters) who have put themselves in random fields without interest and regretted their experience dearly. 

As for your promise, it is advisable to state your reasons as in your question to your previous supervisor. But the decision remains yours.