Changing the thesis topic and supervisor I am currently doing my Master's degree in Computer Science. So far, I am at the very beginning of my work, just started with the research. My supervisor is the same person who supervised me during my Bachelor's. I was very satisfied with collaboration with him. The idea for my Master's was to build on the grounds of the application that was build during my Bachelor's. That was planned from the start of our collaboration during my Bachelor's.

To be honest, I was not very satisfied with the application created for Bachelor's thesis, because I was working and doing double degrees at the same time and created it in a big hurry. Furthermore, I believe that I was able to defend my thesis on time hugely because of my promise to improve the application further during my Master's.

Here comes the problem - I am no longer interested in my Master's thesis topic that much. I came up with another idea, which would include changing the field of research, hence, changing my supervisor. The new idea makes me way more excited, because it is associated with my hobby. I believe that changing the topic would dramatically increase my motivation resulting with a better outcome. All of that makes the decision quite tough for me.

To sum up - is it okay to abandon my supervisor and the improvement of my application written for my Bachelor's despite the promise? Thank you very much in advance.
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.