You can update a game on Google Play as often as you like - so if you want to amend the game etc. you can do so.
As long as your description is clear i.e. here are the first 3 chapters / levels and more will be released as they're completed, users should be okay with that.
If you don't specify that, then they'll most likely ask for a refund as they might feel they paid for something they didn't get.
Another solution would be to have in-app purchase and you could release them in blocks like:
- Chapter 1 or Level 1 is free
- and then charge $1 for every additional chapter, or every additional 2 chapters depending on your price-point. That way you're free to add in chapters, and people can buy them or not as they wish and they won't feel that they've paid for something they haven't received.
Or split your project into versions :
- LongStory 1 - is the first 5 chapters or levels that they buy separately
- LongStory 2 - is the next 5 chapters or levels that they buy separately etc.