NTA will follow the same syllabus and books while conducting NEET. Even if they come up with some changes, it won’t be that drastic. However, it is better to be prepared in a way that you are able to face any challenge. Finish the NCERT books properly and clear your basics. For tougher questions and a better score, use reference books. The method of preparation will hardly change for NEET 2019. 

As for your question about percentage of questions - no data is available online for the changes in paper pattern, I would suggest you ask some coaching institute for exact details as they are well aware of the question pattern and weightage. NCERT has always been the foundation for NEET, so definitely do it well. In an analysis done, of 90 questions of Biology in NEET 2017, 76 were from NCERT. You can find the details of this study in the link below:

https://www.hindustantimes.com/education/neet-2017-coaching-gurus-in-kota-analyse-the-exam-question-paper/story-1OpFAI9S3d8GEkx6luoibO.html

So therefore, it is safe to assume that NCERT will continue to be an important resource for NEET 2019 preparation, so do not neglect it. The books are simple and easy to understand, with good examples and explanations. Definitely study them well for NEET 2019. 

 Hope this helps, good luck!