I'm not a patent lawyer and don't have much experience with provisional patent applications, but I do have quite a few patents some of which are software related. I'm going to try to answer your questions, but don't consider this legal advice.
What should I include in the drawings?
You should include whatever is necessary to describe your invention. This might be drawings, charts, text or even equations.
Are screenshots of my software acceptable?
I have patents which include screenshots so I'm sure it is OK.
What should I include in my flowcharts?
Flowcharts, should you include them should help describe the invention. It is up to you to decide what they include.
How specific should the flowcharts be?
Specific enough to help describe the invention. I'd avoid being so specific that it only shows a single implementation as that might limit your ability to gain a broad enough patent.
If needed, can I change my drawings when applying for the non-provisional patent?
The non-provisional application can be a completely different document. That said, if you want to use the filing date of the provisional application, then the provisional has to describe the invention sufficiently. If you add a new aspect of the invention in the non-provisional application, I believe you might potentially lose the earlier date.
You really should consult with an attorney about this. Obtaining patents is a complex process and software related patents even more so. It is very important not to just patent your specific implementation as that will likely leave a lot of room for someone to circumvent your patent. You need to consider as many ways as you can to achieve the same result, even if they don't seem quite as good as your particular implementation. Drafting applications and especially claims which achieve sufficiently broad protection is a skill which few, if any, lay people have.